KRT40 is a type I keratin protein involved in hair and skin differentiation. It is expressed in the hair shaft, particularly in the fiber cuticle 1, and plays a role in late hair differentiation as part of the keratin intermediate filament network essential for hair structure. In hair follicle engineering, KRT40 is differentially expressed in the inner portion of reconstructed hair follicle-like structures 2, suggesting functional involvement in follicular epithelial organization. Beyond dermatological function, KRT40 is expressed in kidney tissue, where genome-wide association studies identified common variants associated with urinary uromodulin excretion levels 3. Modulation of KRT40 expression in thick ascending limb cells affects uromodulin processing, indicating a kidney-specific functional role. Network analysis in alopecia areata identified KRT40 as a hub gene associated with hair follicle pathobiology 4. Additionally, KRT40 expression is disrupted by environmental toxicants; PFOS exposure during early skin development altered KRT40 expression among other keratinocyte-related genes and impaired developmental signaling pathways 5. These findings suggest KRT40 functions as a structural protein in epithelial differentiation with tissue-specific roles in both hair and kidney physiology.