Tumors arising in CD137L^−/− mice are B-cell lymphomas of clonal origin. (A) Flow cytometric analysis of tumor cells isolated from spleen and stained for CD19 and GL7 with mAbs directly conjugated to PE or FITC, respectively. Mean fluorescence intensity is represented for each event analyzed. Forward and side scatter gating was set to focus on tumor cells. (B) Diagnosis of tumor clonality by PCR assessing the relative location of rearranged JH1, JH2, JH3, and JH4 segments from V(D)J junctions. Splenic B cells from a WT mouse were used as control for a polyclonal B-cell population. Tumors were in advanced stages and isolated from spleens and liver (case 4) of aged (13-16 months) (CD27^−/−); CD137L^−/− mice. Further characteristics of these tumors are depicted in Table 1.