Real-time quantitative RT-PCR analysis on sorted cell compartments from RIP-Tag2 tumors demonstrated Arf expression, as expected, in the tumor cell compartment, but not in endothelial cells (CD31+), immune cells (CD45+), or pericytes (Pdgfrβ+) (Figure 6A and S4). The gene discussed is PDGFRB; the disease is neoplasm.