Fig. 2. Retroviral vectors used. The constructs are based on the ts21 version of E26 virus containing a temperature-sensitive point mutation in v-Myb. At the 3′ end of the coding sequence, an IRES-driven expression cassette was added to create the viral vectors (A) ts21-MafB, expressing full-length MafB, (B) ts21-DNMafB, expressing an N-terminally truncated dominant-negative allele of MafB, and (C) the ts21 vector control construct containing the IRES element only.