Differential expression of endogenous mouse mammary tumor virus genes during development of the BALB/c mammary gland.

Expression of endogenous mouse mammary tumor virus sequences varied over the course of development of the mammary gland during primary pregnancy and lactation in virus-free BALB/c mice. Although RNA from all regions of the genome was detected, both the level and temporal regulation of expression were different for long terminal repeat-, env-, and gag-pol-specific RNAs. Analysis of the methylation status of proviral DNA indicated differential accessibility of the three endogenous units during development. The results demonstrated noncoordinate regulation of mouse mammary tumor virus expression with respect to provirus template utilized and specific transcripts accumulated.