The mouse sex-limited protein (Slp) gene is dependent on androgen for expression, unlike its homologous neighbor, which encodes the fourth component of complement (C4). We have found that the extensive identity of Slp and C4 is disrupted by an endogenous provirus inserted 2 kb upstream of Slp. The 5' LTR of this element corresponds to the previously characterized hormone-responsive enhancer associated with Slp regulation, leading to the conclusion that the provirus has conferred androgen response on the adjacent Slp gene. The provirus is extremely old, based on LTR sequence divergence, the accumulation of mutations in former retroviral-like coding regions, and its stability within the mouse genome. The association of this transposable element with Slp regulation thus provides a long-sought example of an insertional mutation that has been maintained in evolution.