MLH1 and mismatch repair cancer syndrome 1: In this paper, we address the question of whether MMR deficiency and MSI are present in avian tumors induced with avian leukosis virus subgroup A. We describe the status of MSI, the MMR system (MSH2 and MLH1), the cell cycle and apoptosis in DF-1 cells infected with avian leukosis virus subgroup A. Our data revealed that infection with ALV RAV-1 could cause confusion in the cell cycle and increase the expression of MSH2 and MLH1 mRNA.