RAC1 and leukemia: Similar to NTAL, CD44 serves as a platform for signal transduction (35, 36) for PKN2, Rho GTPases RAC1 and RHOA, Rho kinases, and phospholipase C. CD44 is highly expressed in many cells and most abundantly in cells of the hematopoietic system and is also known as a stem cell marker, first described for HSCs and later on confirmed for cancer- and leukemia-initiating cells (37).