HERV-K human endogenous retroviruses show up-regulated expression in tumors. In particular, splicing events in the env region generate a series of transcripts which utilise the +2 reading frame, relative to the env reading frame. The proteins show activity typical of transcriptional regulators, and they also have oncogenic potential. Two related proteins, PCAP2 and PCAP3, are strongly associated with breast cancer and prostate cancer, respectively. PCAP4 stimulates cell division. These proteins can be used in cancer diagnosis and therapy, and are also drug target e.g. for adjuvant therapy. The identification of these splice products is remarkable because full sequence information has been available for HERV-K viruses since 1986.