(a) Ikzf1-/- pro-B cells were successively transduced with MIG-Rag2 and MIRRag1 or with MIGR1 and MIR1 as controls. Genomic DNAs from indicated cells were analyzed as in Fig. 1d, 3b to determine Igh rearrangement status. (b) Analysis of Igh germline transcripts. Top, a schematic of the Igh locus with the different germline transcripts denoted VHJ558, VHQ52, VH7183, μ0 and Iμ. Bottom, RT-PCR analysis of Igh germline transcripts in Rag2-/- and Ikzf1-/- pro-B cells. Serial dilutions of cDNA were analyzed after normalization to Hprt1. All data are representative of at least two independent experiments.