MAP2K1 and hairy cell leukemia: Indeed, in these BRAF wild-type patients, leukemic cells often carried a particular unmutated or lowly mutated immunoglobulin heavy chain variable gene rearrangement (IGHV4-34)39 that is typical of HCL-variant,40 a mimicker of HCL that responds poorly to purine analogs.41 Consistently, the unmutated or lowly mutated IGHV4-34 rearrangement associates with clinical and genetic features similar to those of HCL-variant, including higher WBC counts at diagnosis, low response to cladribine, and frequent activating mutations of MAP2K1/MEK140,42,43 (encoding the kinase phosphorylated by BRAF).