Genetic loci responsible for the encoding of minor lymphocyte stimulatory antigens. There are at least two unlinked loci (in the mouse) and they appear to be separate from the MAJOR HISTOCOMPATIBILITY COMPLEX and MINOR HISTOCOMPATIBILITY LOCI. The mouse mammary tumor virus (see MAMMARY TUMOR VIRUS, MOUSE) has the ability to integrate into these loci. The antigens induce strong T-cell proliferative responses in mixed lymphocyte reactions.