These observations suggest a significant contribution of SOS1 to CML development and support the notion that, as it happens with various other cellular functionalities previously characterized in our single and double SOS1/2-KO strains [19,26,27,31,35], SOS2 may just play a partially overlapping or ancillary role relative to SOS1 with regard to the development of CML. This evidence concerns the gene SOS1 and chronic myelogenous leukemia, BCR-ABL1 positive.