Reverse transcriptase-polymerase chain reaction analysis of sera from measles mumps rubella (MMR) vaccine recipients for the presence of avian leukosis virus (ALV) (A) and endogenous avian retrovirus (EAV) (B) RNA. Lanes 1-10, samples from MMR vaccine recipients. NC, negative control, uninfected human serum; PC, positive control, culture supernatant from Rous-associated virus 0 (RAV-0) infected 15B1 chick embryo fibroblasts.