Hoxc13 belongs to the Hox family of homeobox transcription factors, master regulators of embryonic patterning that specify body plans along the head-to-tail axis. Within this family, Hoxc13 has long been recognized as a critical controller of integumentary appendages, the skin-derived structures that include hair, nails, and claws. Previous work by the same team had shown that during mammalian evolution, the Hoxc13 protein acquired a characteristic polyglycine repeat insertion that distinguishes it from its non-mammalian homologs, the related proteins found in birds, reptiles, and other vertebrates. The new study set out to test whether this small structural addition actually changes what the protein can do at the genome-wide level, and whether that change could plausibly have contributed to the emergence of hair.

To answer this question, the researchers employed an elegant experimental design built around two versions of the Hoxc13 protein from cashmere goats. The first, designated Hoxc13-W, is the wild-type protein carrying the intact polyglycine repeat. The second, Hoxc13-S, is a shortened variant in which the polyglycine fragment has been deleted, mimicking the ancestral, non-mammalian form of the protein. Both proteins were synthesized using an in vitro expression system, allowing the team to compare their DNA-binding behavior directly without the confounding effects of living cells. This reductionist approach isolates the intrinsic DNA-binding capacity of each protein variant, a critical consideration when the goal is to attribute functional differences to a specific structural feature.

The centerpiece of the study was DNA affinity purification sequencing, or DAP-seq, a technique that uses purified proteins to fish out genomic DNA fragments they can bind, which are then identified by high-throughput sequencing. The results were striking. Hoxc13-W, the version with the polyglycine repeat, identified 12,679 binding peaks across the goat genome. Hoxc13-S, lacking the insertion, identified only 3,634 peaks. Even more telling was the overlap: just 243 peaks were shared between the two proteins. In other words, the vast majority of genomic sites bound by the wild-type protein were not recognized by the polyglycine-deleted variant, and vice versa. A single short amino acid insertion had fundamentally rewired the DNA-binding landscape of this transcription factor, transforming it from a relatively restricted regulator into one with a vastly expanded reach.

Functional enrichment analysis of the genes associated with these binding peaks revealed why this expansion matters for hair. The targets of Hoxc13-W were specifically enriched in pathways governing hair follicle morphogenesis and cycling, the developmental programs that build follicles and drive them through their growth, regression, and resting phases. By contrast, the targets of Hoxc13-S clustered primarily in the axon guidance pathway, a set of genes involved in steering neuronal growth cones rather than in skin biology. This shift in target preference suggests that the polyglycine insertion did not merely add more binding sites but redirected the protein toward an entirely different set of biological functions, aligning its regulatory activity with the demands of building hair-producing skin.

One of the most consequential findings concerned the Wnt signaling pathway, a cascade of intercellular communication that plays a central and well-documented role in hair follicle development, stem cell activation, and the hair growth cycle. Within this pathway, Hoxc13-W bound 30 genes, while Hoxc13-S bound only 5. The sixfold difference in Wnt pathway coverage implies that the polyglycine-bearing form of Hoxc13 can exert far broader control over one of the most important signaling networks in hair biology. Because Wnt signaling is repeatedly recruited at multiple stages of follicle morphogenesis and regeneration, an expanded capacity to regulate Wnt pathway genes could have provided mammalian ancestors with finer and more powerful control over hair production.

The study also probed how the polyglycine repeat affects the recognition of cis-regulatory motifs, the short DNA sequences to which transcription factors attach. Motif analysis demonstrated that the polyglycine fragment substantially influenced the motifs associated with Hoxc13 binding beyond the core Hoxc13-binding motif, which has the consensus sequence 5′-ATAAA-3′. This suggests that the insertion does not simply strengthen or weaken binding to the same sequence; rather, it changes the sequence context in which the protein operates, likely through cooperation with other DNA-bound factors or through altered preferences for flanking sequence features. Such changes in motif usage can ripple through regulatory networks, opening or closing access to large suites of target genes across the genome.

Molecular docking analysis added a structural dimension to the story. The computational modeling indicated that the polyglycine fragment alters the N-terminal structure of the Hoxc13 protein, the region involved in protein-protein interactions. Glycine is the smallest amino acid and confers exceptional conformational flexibility, so a repeat of glycine residues can act as a flexible hinge or spacer within a protein. By reshaping the N-terminal architecture, the insertion appears to affect how Hoxc13 interacts with partner proteins, which in turn influences how it engages DNA in a cellular context, since many transcription factors bind DNA effectively only as part of larger protein complexes. The polyglycine repeat, in this view, functions as a gain-of-function modification that enhances both the DNA-binding breadth and the interaction capacity of the protein.

Taken together, the findings support a model in which the polyglycine repeat insertion in Hoxc13 acted as an evolutionary gain-of-function mutation that amplified the Hoxc13-mediated gene regulatory network during the transition from non-mammalian to mammalian vertebrates. Rather than requiring wholesale changes in gene sequence or the emergence of entirely new genes, this scenario illustrates how a modest insertion of flexible, low-complexity sequence within a single transcription factor can reconfigure its binding profile, redirect its pathway targets, and reshape its protein interactions. Such low-complexity insertions are increasingly appreciated as a rich source of evolutionary novelty, capable of tuning protein behavior with relatively small mutational steps.

The research, conducted by Guanghao Yang, Chaoyong Huang, and colleagues at Inner Mongolia Minzu University, also carries practical implications beyond evolutionary biology. Cashmere goats, the species from which the two protein variants were derived, are prized for their fine undercoat fibers, and a deeper understanding of the Hoxc13-centered regulatory network could inform breeding strategies aimed at improving fiber quality and yield. More broadly, the study demonstrates the power of combining DAP-seq with pathway enrichment, motif analysis, and molecular docking to dissect the functional consequences of protein sequence evolution. While the polyglycine repeat is certainly not the sole driver of mammalian hair, the evidence presented here makes a compelling case that this tiny stretch of glycine helped tip the balance toward the furry integument that now defines the class Mammalia, from the wool of sheep to the whiskers of cats to the hair on our own heads.

Subject of Research: The role of a Hoxc13 polyglycine repeat insertion in mammalian hair and integument evolution

Article Title: Hoxc13 polyglycine repeat gain-of-function contributes to mammalian integument evolution by altering targeted genes and interactions

Article References: Yang, G., Huang, C., Yu, M., Li, W., Wu, D., Li, C., Hu, S., & Wu, J. (2026). Hoxc13 polyglycine repeat gain-of-function contributes to mammalian integument evolution by altering targeted genes and interactions. BMC Genomics. https://doi.org/10.1186/s12864-026-13314-5

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13314-5

Keywords: Hoxc13, polyglycine repeat, hair follicle, integument evolution, transcription factor, DAP-seq, Wnt signaling, gene regulation, mammalian evolution, cashmere goat, molecular docking, BMC Genomics