KRTAP3-3 (keratin-associated protein 3-3) is a structural protein essential for hair shaft formation. In the hair cortex, KRTAP3-3 functions as part of the interfilamentous matrix surrounding hair keratin intermediate filaments, contributing to hair rigidity and resistance through extensive disulfide bond cross-linking with cysteine-rich hair keratins [UniProt annotation]. The protein exhibits protein binding activity and localizes to the cytosol. Genomically, KRTAP3-3 shares regulatory elements with related family members KRTAP3-1 and KRTAP3-5 in skin tissue, suggesting coordinated expression patterns 1. Beyond its primary hair structural role, recent genetic studies have identified KRTAP3-3 as associated with chr17 (circadian preference) in a large-scale exome-wide association study of 450,000 UK Biobank participants, with findings validated in the FinnGen cohort 2. Clinically, KRTAP3-3 has emerged as a potential biomarker in head and neck squamous cell carcinoma (HNSCC), identified through machine learning analysis as a gene with significant differential expression in cancer tissues and prognostic value 3. These findings suggest KRTAP3-3 may have roles extending beyond hair biology, though the mechanistic basis for its association with sleep-wake timing and cancer biology remains to be elucidated.