Class II human endogenous retroviruses (HERVs), often referred to as mouse mammary tumour virus (MMTV)-like or HERV-K elements, have similarities to several animal infectious retroviruses. Single clones from each of nine class II HERV groups (NMWV 1 to NMWV 9), isolated from a human breast cancer cell genomic library, were sequenced over a 244 bp stretch of the conserved reverse transcriptase region. These sequences were aligned to related exogenous and endogenous retroviruses and a phylogenetic tree was constructed. Sequences with more than 80% identity were considered as members of one group and we report here that the class II HERV family consists of at least ten groups. Three of the sequenced clones, from groups NMWV 3, 7 and 9, could not be related to any other previously identified elements and constituted their own groups. NMWV 8 had no similarity to any retroviral sequences in the sequenced region and is so far considered to be non-retroviral.