RPTN (repetin) is a multifunctional S100 fused-type protein primarily involved in cornified envelope formation and epidermal barrier function 1. The protein contains functional EF-hand domains that reversibly bind calcium and is localized to keratohyalin F-granules in the stratum granulosum 1. RPTN is expressed throughout the epidermis with particularly strong expression in the acrosyringium and inner hair root sheath 1. UVB irradiation significantly increases RPTN expression in the granular layer, suggesting a role in epidermal barrier repair through ERK1/2 and STAT3 pathway activation 2. In atopic dermatitis (AD), RPTN expression is elevated in both lesional and nonlesional skin, and specific polymorphisms (rs3001978) are associated with disease severity, early onset, and concomitant asthma 34. Beyond dermatology, RPTN is ubiquitously expressed in the brain, particularly in the hippocampus and prefrontal cortex, with significantly diminished serum levels in patients with schizophrenia and bipolar disorder, suggesting involvement in emotional and cognitive processing 5. These findings indicate RPTN functions as both a structural barrier protein and a potential biomarker for neuropsychiatric disorders.