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The Janus kinase (JAK)-signal transducers and activators of transcription (STAT) pathway is an important signaling pathway of interferons and cytokines. We examined the activation of STAT proteins induced by interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), or erythropoietin (EPO) using the human leukemia cell line, UT-7, which requires these cytokines for growth. IL-3, GM-CSF, and EPO induced DNA-binding activity to the oligonucleotides corresponding to the sis-inducible elements (SIE) of c-fos, in addition to the β-casein promoter (β-CAP), SIE- and β-CAP-binding proteins were identical to Stat1α and Stat3 complex and to Stat5 protein, respectively. This indicates that IL-3, GM-CSF, and EPO commonly activated Stat1α, Stat3, and Stat5 proteins in UT-7. However, EPO hardly activated Stat1α and Stat3 in UT-7/GM, which is a subline of UT-7 that grows slightly in response to …