Sperm-specific Na(+)/H(+) exchanger involved in intracellular pH regulation of spermatozoa (PubMed:20036903). Involved in sperm motility and fertility (PubMed:20036903, PubMed:27010853, PubMed:19409551). Testis-specific (PubMed:20036903). Expressed in the spermatids and spermatozoa (at protein level) (PubMed:20036903). Specifically present in the principal piece of sperm tail (at protein level) (PubMed:20036903, PubMed:27010853, PubMed:19409551). Mice are normal but males shown reduced fertility caused by diminished sperm motility (PubMed:27010853). Addition of cAMP analogs almost completely rescue the motility and infertility phenotypes in vitro (PubMed:27010853). Double knockout of SLC9B1 and SLC9B2 results in male infertility with a severe sperm mobility reduction, indicating that these two gene are functionally redundant (PubMed:27010853). Belongs to the monovalent cation:proton antiporter 1 (CPA1) transporter (TC 2.A.36) family.