CD38 and acute lymphoblastic leukemia: Despite the presence of variable amounts of circulating CD45+/CD34+/CD38− stem cells, in none of these PB samples studied a co-expression of CD26 was recorded [Figure 3 shows the absence of circulating CD26+ LSCs in a representative IMF patient (a, b), Ph+ ALL patient (c, d), AML patient (e, f), and healthy donor (g, h), respectively].