Probable transporter, its substrate specificity is unknown. Localizes to the midpiece of the sperm tail. Widely expressed at low level (PubMed:12801629, PubMed:16141072, PubMed:17472575). Highly expressed in testis by Sertoli cells and Leydig cells (PubMed:12801629, PubMed:16141072). Detected in testicular germ cells and sperm (at protein level) (PubMed:17472575). First detected at 3 weeks of age IN the pachytene spermatocytes. During germ cell differentiation in the adult testis, pachytene spermatocytes in stage VI of the epithelial cycle are the first germ cells to show MRP9 expression. Belongs to the ABC transporter superfamily. ABCC family. Conjugate transporter (TC 3.A.1.208) subfamily. Does not transport any of the organic anions transported by the other multidrug resistance-associated proteins (MRPs) in vesicular transport assays, nor does it confer resistance to cytotoxic agents in intact cell assays.