Pax2 is a transcription factor encoded by a gene that is required for the embryonic development of a variety of mammalian tissues, including the kidney, optic cup, and inner ear. Structurally, the Pax2/5/8 protein subfamily is characterized by an N-terminal paired domain, a conserved octapeptide sequence, and a paired-paired type homeobox. The N-terminal paired domain of the Pax2/5/8 proteins binds DNA, while the C-terminal region is required for activation of target genes. In human mammary tissue, Pax2 expression has been identified in sub-populations of mammary ductal cells, and also in > 50% of human breast tumors surveyed (n=38). Mouse studies have demonstrated co-localization of Pax2 and WT1 (Wilms’ tumor suppressor protein) in the nuclei of normal and cancerous breast cells, indicating a role for Pax2 in the regulation of the response to progesterone in the mature mammary gland. Pax2 is a phosphorylation target of JNK, and has been observed to co-imunoprecipitate with JIP1. Phosphorylation of Pax2 enhances its ability to transactivate reporter genes.