KRT33A encodes keratin 33A, a type I intermediate filament protein that serves as a structural constituent of hair and skin epidermis. It is expressed primarily in the cortex of hair fibers 1, where it contributes to intermediate filament organization and cytoskeletal architecture. During hair follicle development, KRT33A expression is upregulated in keratinocyte-dermal papilla constructs, particularly when exposed to developmental signaling pathways including Wnt, BMP, FGF, and Shh activation 2. The gene's expression pattern in reconstructed hair follicles suggests a role in hair follicle morphogenesis and epithelial differentiation 2. Beyond developmental contexts, KRT33A has forensic applications as a reliable polymorphic marker; it was successfully incorporated into a mass spectrometry-based mRNA typing system for human identification from single hair shafts, with genotyping results concordant with DNA typing 3. The gene's presence in hair shaft transcripts across diverse sample conditions (aged, white, permed, and dyed hair) demonstrates its stable expression in mature hair structures 3. These findings establish KRT33A as both a functionally important hair structural protein and a forensically informative biomarker.