As the youngest of the endogenous retroviruses, it is not surprising to find that HERV-K (HML-2) is the most transcriptionally active of the endogenous retroviruses [13-19], and under certain circumstances expression of HERV-K (HML-2) RNA, viral proteins, and virus-like particles (VLPs) can be seen, especially in breast cancer, melanoma, and teratocarcinoma cell lines, although these particles have appeared to be incapable of infection [15,20-23]. This evidence concerns the gene CLEC10A and breast cancer.