Transgenic mice expressing human urokinase, as well as animals expressing human urokinase receptor under the control of the murine mammary tumor virus (MMTV) long terminal repeat, were established. In the vast majority of the founder animals and their descendants, the transgene was completely methylated, corresponding to down-regulation of transgene expression in the mammary gland. Two lineages with human urokinase receptor as the transgene with mixed methylation of the transgenes were analyzed in more detail. We show here for the first time that the methylation status of the transgene is identical in different organs of an animal, but may differ from animal to animal among the descendents, In the mammary gland, complete methylation of the transgene was incompatible with expression; unmethylated and mixed methylation transgenes gave rise to expression at the RNA as well as at the protein level. The methylation observed was not the consequence of an imprinting process, Surprisingly, in organs other than the mammary gland, such as liver, kidney and spleen, weak expression of the transgene was noted independent of the methylation status of the MMTV promoter. With respect to the molecular mechanism it is unresolved whether the human growth hormone sequence of the transgene harbors a methylation inducing element responsible for the observed methylation pattern.