Provirus was present in 89% of human myeloid colonies derived from xenografted TBM-BGI mice. CFU-GM colonies were picked and PCR analysis performed on DNA using vector-specific primers: lane 1, NBM; lane 2, TBM; lanes 3 to 29, TBM-BGI; and lane 30, Φ. Vector-specific bands were seen only in the CFU-GM colonies (in 24 of 27 colonies) derived from the TBM-BGI xenografts.