Since only IgM BCRs from humans and mice with CLL can signal autonomously (Figure 4, A and B), the lack of CD19+CD5+IgD+ CLL-like cells supports our contention that IgM-specific autonomous signaling is required for the overgrowth and accumulation of leukemic B cells in TCL1 mice. Here, CD5 is linked to B-cell chronic lymphocytic leukemia.