To identify transforming genes in acute myeloid leukemia (AML) we here constructed a retroviral cDNA expression library from an AML patient, and then used this library to infect a mouse cell line 32Dcl3-mCAT. cDNA inserts of the cell clones which proliferated in the presence of granulocyte colony-stimulating factor were derived from JAK3 encoding a JAK3 mutant with a valine-to-alanine substitution at codon 674 and two additional amino acid substitutions. The transforming activity of JAK3(V674A) was confirmed by its introduction into 32Dcl3-mCAT. Sequencing of the original JAK3 cDNA derived from the patient, however, failed to detect the V674A mutation.