CD93 and lymphoma: As shown in Table 3, we found that, with one exception (mouse 353), all of the lymphoma cells stained positive for CD19, CD45R/B220 and high AA4.1 (CD93) expression and negative for CD5 and CD3, indicating that they are early stage B-cells, either surface IgM− or IgM+, in both Mtap+/+ and MtaplacZ/+ mice.