In vivo BrdU incorporation assays and Annexin V staining revealed a several fold increased proliferation and apoptosis rate of CD19+CD5+ CLL cells from TCL1 Nfat2−/− mice as compared to TCL1 mice with intact NFAT2 expression demonstrating a highly accelerated tumour cell turnover (Fig. 2e). Here, CD5 is linked to B-cell chronic lymphocytic leukemia.