Somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin genes are initiated by the enzymatic deamination of cytosine to uracil. Uracil-DNA-glycosylase (Ung2) converts uracils into apyrimidinic sites, which is essential for the generation of G/C transversions during SHM and efficient DNA break formation during CSR. Besides Ung2, the mismatch repair protein Msh2 and the translesion synthesis DNA polymerase Rev1 are implicated in SHM and CSR. To further unravel the role of Rev1 we studied wild type, Rev1-deficient, Msh2-deficient, and Rev1/Msh2 double-deficient B cells. Loss of Rev1 only slightly reduced CSR. During SHM G/C to C/G transversions are generated in Ung2- and Msh2/Ung2-dependent fashions. We found that Rev1 is essential 0for the Msh2-independent generation of these transversions downstream of Ung2-induced AP sites. In the Msh2/Ung2 hybrid pathway Rev1 is not essential and can be substituted by an alternative TLS polymerase, especially when Rev1 is lacking. This article is protected by copyright. All rights reserved.