SOS2 and chronic myelogenous leukemia, BCR-ABL1 positive: The differences in the number, growth rate, and size of the HSPC colonies arising from the SOS1-KO or SOS2-KO transgenic mice suggest that these two GEF isoforms act at different levels of stem cell renewal and proliferation and are also consistent with the prevalent role shown by SOS1 in our previous assays of CML development in these CML-prone mouse strains.