Lck belongs to the Src-like non-receptor tyrosine kinase family with the typical Src family kinase structure: a unique amino terminal domain (Src homology 4 domain, SH4) followed by an SH3 domain, an SH2 domain, a kinase domain (SH1), and a carboxy-terminal negative regulatory domain. Lck activity is controlled by the interactions of SH2 and SH3 domains as well as tyrosine phosphorylation status of the activation loop. Lck is recruited to the T cell receptor (TCR) complex upon stimulation and activates downstream tyrosine kinases to initiate T cell signaling. Lck is also found to be involved in the regulation of mitochondrial apoptosis pathways and may be responsible for some anticancer drug induced apoptosis.