CrkL is a 39kD adaptor protein, originally cloned in proximity to the BCR gene on chromosome 22, which has a key regulatory role in hematopoietic cells. CrkL has one SH2 and two SH3 domains, with 60% homology to CrkII. Structurally, the amino-terminal SH3 domain of CrkL has been shown to bind proteins such as C3G, SOS, PI3K, c-Abl or BCR/Abl. The SH2 domain of CrkL can bind to tyrosine-phosphorylated proteins such as Cbl, HEF1, CAS or paxillin. CrkL is involved in various signalings initiated by different cytokines and growth factors. The biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration, immune cell responses and so on. CrkL is a prominent substrate of the BCR/Abl oncoprotein in chronic myelogenous leukemia and binds to both BCR/Abl and c-Abl. CrkL is prominently and constitutively tyrosine phosphorylated in CML neutrophils and is not phosphorylated in normal neutrophils. Moreover, stimulation of normal neutrophils with cytokines and agonists does not induce tyrosine phosphorylation of this protein, indicating that it may be a useful target for therapeutic intervention or as a disease marker. Tyrosine 207 in CrkL is the BCR/Abl phosphorylation site.