The SIVmac239 infectious clone does not have a premature stop codon in its transmembrane protein (TMP) region and it produces full-length (41 kilodalton, kDa) TMP in macaque peripheral blood lymphocytes (PBL) in vitro and in vivo. However, viruses with truncated forms of TMP (28kDa) are selected during propagation in human cell types; truncated forms arise from point mutations, CAG (glutamine) to TAG (stop), in the viral genome. These results document molecular changes associated with adaptation of SIVmac for growth in human cells.