Gfi1b regulates transcription of a large number of genes in AMuLV pro–B cells. (A) Hierarchically clustered heat map of genes whose transcription level is significantly altered (P < 0.01) upon retroviral expression of Gfi1b in RAG1-GFP^High cells. Each column represents an independent replicate. (B, top) Quantitative RT-PCR analysis of germline Igκ and SpiB transcripts relative to HPRT in RNA from Gfi1b-ER–overexpressing AMuLV cells treated with tamoxifen (4-OHT) as indicated. (B, bottom) Quantitative RT-PCR analysis of these same transcripts relative to HPRT using RNA from wt or Gfi1b^−/− AMuLV cells. Error bar represents standard deviation of triplicate PCR assays.