This proposal for the GenitoUrinary Development Molecular Anatomy Project (GUDMAP) will use the mouse model to investigate how differences between male and female external genitalia arise at the single cell level. Congenital malformations of the external genitalia (CAEG) are among the most prevalent human birth defects, affecting ~1:150 live male births. Hypospadias is a CAEG that is characterized by ectopic opening(s) of the urethra on the ventral side of the penis. In severe hypospadias, the urethral plate can open along the entire underside of the penis, giving it a clitoris-like appearance. This condition is termed ambiguous genitalia. Genetic causes of hypospadias have been elusive. The global prevalence has led to increased scrutiny of environmental factors, particularly endocrine disrupting chemicals (EDCs) that can induce genital anomalies in animal models. Sexual differentiation of the embryonic genital tubercle (GT) into a penis or a clitoris is regulated by gonadal androgens and estrogen. The central role of sex hormones in GT development creates sensitivity to EDCs; however, the mechanisms that mediate EDC effects on the GT are unknown, particularly at a cellular level. Do EDCs that cause hypospadias act on all cells in the GT, or do they target specific cell types? If subpopulation(s) of hormonally-responsive cells control urethragenesis, then buffering those cells against EDCs could be a method for prevention of hypospadias (analogous to folic acid’s ability to prevent neural tube defects). For genetic hypospadias, identifying cell-specific gene functions can distinguish direct regulators of urethragenesis from genes with indirect effects (e.g. sensitivity to EDCs). Understanding cell type diversity in developing external genitalia is essential for identifying the causes of CAEG and for developing preventative strategies. A major obstacle to progress in prevention and treatment of CAEG is the lack of knowledge of the specific cell types in external genital tissues. This project aims to fill these critical knowledge gaps by using single cell RNA-sequencing combined with novel imaging modalities to map cell type diversity in the developing external genitalia of normal male and female mice.