Based on limited published evidence, KRTAP9-4 encodes a keratin-associated protein that functions in hair shaft structural integrity. According to UniProt annotations, KRTAP9-4 is embedded within the hair cortex matrix and participates in extensive disulfide bond cross-linking with hair keratins to form rigid, resistant hair shafts [UniProt]. The protein exhibits cytosolic localization and protein-binding capability. A recent exome-wide association study identified a KRTAP9-4 variant in the 3' untranslated region as significantly associated with suicidal ideation in a veteran cohort 1, though the biological relevance of this association to the protein's known hair structural role remains unclear.