SCGB1D2 (secretoglobin family 1D member 2) is a secreted protein that functions as a host defense factor and potential steroid-binding molecule. The protein demonstrates antimicrobial activity, specifically inhibiting growth of Borrelia burgdorferi both in vitro and in vivo mouse models, suggesting it protects against Lyme disease infection 1. SCGB1D2 is present in skin, sweat, and other secretions where it serves as a barrier against bacterial pathogens 1. The protein frequently co-expresses with mammaglobin A (SCGB2A2) in various tissues, forming complexes particularly in breast and gynecologic tissues 2. Expression patterns show tissue-specific regulation, with concordant expression of SCGB1D2 and mammaglobin A in breast, endometrial, ovarian, and cervical cancers 2. In cancer contexts, SCGB1D2 shows variable expression patterns - it can be overexpressed in histologically normal breast epithelium adjacent to tumors 3 and its presence correlates with tumor ulceration in melanoma 4. The protein may serve as a potential biomarker for certain cancers, though its clinical utility appears broader than initially thought for breast-specific applications 2. Genetic variants in SCGB1D2 can affect its antimicrobial function, with certain missense mutations reducing its protective efficacy against bacterial infections 1.