The human testis-determining gene was recently isolated from a 35 kb region on the human Y chromosome which was present in four sex-reversed individuals, three XX males and one true hermaphrodite. One of the XX males and the true hermaphrodite were sibs. A more detailed molecular analysis of these two patients and their family for Y-DNA sequences including the testis-determining gene, SRY was performed. The father was found to harbor two copies of SRY, one on his Y chromosome and the other on his X chromosome located at Xp22 determined by in situ hybridization. Somatic cell hybrids were generated from peripheral blood lymphocytes. Analysis of Y chromosome-negative somatic cell hybrids from the XX male, the true hermaphrodite and their father, revealed that both the X and Y pseudo-autosomal boundaries were present. The present of both boundaries suggests than an unequal interchange of X and Y material occurred with the cross-over breakpoint located within the X pseudo-autosomal region. The paternal SRY-bearing X chromosome was transmitted to two of his children, a 46 XX true hermaphrodite and a 46,XX male. The presence of SRY on an X chromosome associated with two sex phenotypes strongly suggests that the phenotypic variability was caused by differential inactivation of the SRY-bearing X chromosome, thereby influencing SRY expression.