Expressed in the principal piece of the sperm tail, nearest the sperm head. Isoform 1 is specific to germ cells of the testis and localizes to the principal piece of the sperm flagellum. Isoform 2 seems to be expressed mainly in somatic cells of the testis, and is not detected in mature spermatozoa (at protein level). Isoform 2 may also be expressed weakly in brain. May be due to intron retention.