CD48^−/− tumors can reconstitute a nonirradiated host. Eight- to 10-week-old nonirradiated WT or CD48^−/− mice were injected with either 30 000 or 50 000 tumor cells. Mice were monitored for 20 weeks and killed for pathology. At 30 000 tumor cells, no WT mice (n = 3) developed tumors; however, 1 of 3 CD48^−/− mice (n = 3) developed tumors. At 50 000 tumor cells, no WT mice (n = 5) developed tumors, and all CD48^−/− mice (n = 5) developed tumors (P = .007, Fisher exact test). (A) CD19⁻ splenocytes from mice with reconstituted tumors and those without tumors were collected and analyzed for levels of phosphorylated ERK (pERk). Shown here are representative histograms of the analysis of phosphoprotein. (B) CD19⁻ cells from the reconstituted tumors possess significantly higher levels of pERK (P < .001, Student t test) than the normal CD19⁻ splenocytes.