We have continued our investigation of the age-dependent stability of gene control mechanisms. Our approach has been to examine the stability of gene repression by determining if specific genes which are thought to be normally repressed do indeed remain repressed with increased age of an animal. We have examined alpha and Beta-globin, casein, alpha-fetoprotein genes and the set of genes making up the c-type endogenous mouse leukemia virus (MuLV) and the mouse mammary tumor virus (MMTV) in liver and brain of C57BL/6J and AKR mouse strains. Evidence for age-dependent depression of these genes was found with the exception of casein, which remained repressed. More detailed studies were undertaken on the two endogenous c-type of viruses, where both were found to undergo a qualitative type of gene depression (more different types of genes being depressed with increasing age). Most of the viral RNA is not transferred to the cytoplasm, but instead appears to remain in the nucleus where it cannot be translated.