Alteration of the TAL1 gene is the most common genetic lesion found in T-cell acute lymphoblastic leukemia. TAL1 encodes phosphoproteins, pp42TAL1 and pp22TAL1, that represent phosphorylated versions of the full-length (residues 1 to 331) and truncated (residues 176 to 331) TAL1 gene products, respectively. Both proteins contain the basic helix-loop-helix motif, a DNA-binding and protein dimerization motif common to several known transcriptional regulatory factors. We now report that serine residue 122 (S122) is a major phosphorylation site of pp42TAL1 in leukemic cell lines and transfected COS1 cells. In vivo phosphorylation of S122 is induced by epidermal growth factor with a rapid time course that parallels activation of the ERK/MAP2 protein kinases. Moreover, S122 is readily phosphorylated in vitro by the extracellular signal-regulated protein kinase ERK1. These data suggest that TAL1 residue S122 serves as an in vivo substrate for ERK/MAP2 kinases such as ERK1. Therefore, S122 phosphorylation may provide a mechanism whereby the properties of TAL1 polypeptides can be modulated by extracellular stimuli.