HERV-K (HML-2) group, the most recent retroviruses to colonize the human germ line, which is most closely related to the exogenous mouse mammary tumor virus (MMTV) causing breast cancer in mice (Gallahan and Callahan, 1987; Shackleford et al., 1993), has the greatest coding competence and is acknowledged to be the most biologically active class of HERVs (Bannert and Kurth, 2004). The gene discussed is CLEC10A; the disease is breast carcinoma.