ERVW-1 and infection: Our panel of Env pseudotyped viruses displayed the following infection patterns on pMGLs (Fig. 3A) and MDMs (see Fig. 2): (i) M-tropic Env viruses infected at levels similar to that of VSV G virus, (ii) the 4051 T-tropic Env virus infected at low levels close to background, (iii) the 4059 T-tropic virus, with its intermediate phenotype, infected at levels approaching (but not reaching) that of the M-tropic viruses, and (iv) the X4 T-tropic Env virus (C27D1) showed no infectivity.