Simian-T-lymphotropic viruses, also Simian T-cell leukemia viruses (STLVs), are retroviruses closely related to the human sexually and breastfeeding transmissible viruses HTLV. They have subtypes 1 thru 4 as compared to HTLV 1 thru 4, each subtype have their own serovars. Together they comprise PTLVs (Primate T-lymphotropic viruses) A study has shown that STLV-1 Tax and SBZ proteins have similar functions to their counterparts of HTLV-1. STLV-1 is oncogenic in Japanese macaques.
In particular, the HTLV-I/STLV-I history might suggest a simian migration from Asia to Africa not much earlier than 19,500-60,000 years ago.