VavP-MYC lines with a range of transgene expression levels. (A) Differing expression levels of transgenic MYC protein in MYC17, MYC10, MYC10^hom (homozygous for the transgene locus), and MYC12 mice. Western blot of thymocyte lysates prepared from healthy young transgenic and wild-type mice were probed with anti-MYC antibody. Note that expression of endogenous Myc (a slower-migrating, broad, phosphorylated band) is repressed in the transgenic samples. The same filter probed with anti–β-actin is shown as a loading control. (B) Widespread expression of transgenic MYC RNA in MYC10 mice. Semiquantitative RT-PCR analysis of RNA prepared from thymocytes, splenocytes, bone marrow cells, and sorted pre-B cells (B220⁺ IgM^–), B cells (B220⁺ IgM⁺), monocytes (Mac1⁺ Gr1^–), and granulocytes (Mac1⁺ Gr1⁺) from 8-week-old MYC10 mice. Analyses of RNA with the reverse transcription reaction omitted (–RT) confirms specificity of PCR primers for transgene-expressed MYC RNA. RT-PCR reactions from serial 5-fold dilutions of input RNA are shown. Analysis of endogenous β-Actin RNA is shown as a control.