Table 3 displays results of the PCR analysis of mice from the different DKFZ units summed up in four age groups. Virus prevalence ranged from 0% to 76%. Interestingly, infection and therewith excretion with MuAstV was not age related, even in units with open cages, and most likely independent of the presence of specific antibodies as most animals tested were sero-positive. Fecal samples from immunodeficient mice (i. a. NOD scid gamma, Rag2 knockout, Athymic Nude-Foxn1nu, BALB/c Nude, in square brackets) were mostly tested positive by PCR. This evidence concerns the gene RAG2 and infection.