The difference in the disease phenotypes, thymoma vs. T-ALL, paralleled with the extents of endogenous Rap activation; thus C3G-F+ Spa-1-/- HPC-derived T-ALL cells showed much higher RapGTP levels than C3G-F+ B6 HPC-derived thymoma cells. The gene discussed is LRPAP1; the disease is acute lymphoblastic leukemia.