Amphotropic murine leukemia virus (MLV) replicates in cells from various ma
mmalian species, including humans, and is a potential contaminant in MLV ve
ctor preparations for human gene transfer studies. The generation of replic
ation-competent virus is considered less likely with vectors that delete th
e viral transcription elements. This conclusion is based on data obtained i
n rodents, where MLV replication depends on the expression of viral genes u
nder the control of 75-bp enhancer elements in the long terminal repeat. We
demonstrate here that in some human cells replication of amphotropic MLV i
s possible in the absence of these enhancer elements. Replication of the en
hancer-deficient virus MLV(MOA)DeltaE is observed in selected human sarcoma
and B lymphoma lines and proceeds at a lower rate than that of the intact
virus. No insertion of a foreign promoter or enhancer into the long termina
l repeat was detected. Our data suggest the presence of a secondary enhance
r element within the MLV provirus that can in selected human cells mediate
virus transcription and replication in the absence of the 75-bp U3 enhancer
s.