KLK15 (kallikrein-related peptidase 15) is a serine protease encoded on chromosome 19.4 that exhibits trypsin-like proteolytic activity 1. While its physiological substrate remains uncharacterized, KLK15 likely participates in prostate-specific antigen (PSA/KLK3) activation 1. The gene encodes multiple alternatively spliced transcripts with tissue-specific expression patterns, primarily in thyroid, prostate, salivary, and adrenal glands 2, with emerging evidence of expression in hepatic tissue where KLK15 is upregulated in lipogenic pathways 3. KLK15 expression is androgen-regulated 2 and shows cancer-type-dependent prognostic significance: in prostate cancer, elevated KLK15 mRNA correlates with disease aggressiveness and shortened progression-free survival 4, while in breast cancer, higher KLK15 expression independently predicts favorable outcomes 5. Genetic variants in KLK15, particularly rs2659056, associate with prostate cancer aggressiveness and Gleason score through predicted effects on RORα transcription factor binding 6. KLK15 protein has been detected in prostate, colon, thyroid tissues, seminal plasma, and breast milk 7, supporting its utility as a cancer biomarker across multiple malignancies.