STAT5a and -5b (signal transducers and activators of transcription 5a and 5b) proteins play an essential role in hematopoietic cell proliferation and survival and are frequently constitutively active in hematologic neoplasms and solid tumors. Because STAT5a and STAT5b differ mainly in the carboxylterminal transactivation domain, we sought to identify new proteins that bind specifically to this domain by using a bacterial two-hybrid screening. We isolated hTid1, a human DnaJ protein that acts as a tumor suppressor in various solid tumors. hTid1 interacts specifically with STAT5b but not with STAT5a in hematopoietic cell lines. This interaction involves the cysteine-rich region of the hTid1 DnaJ domain. We also demonstrated that hTid1 negatively regulates the expression and transcriptional activity of STAT5b and suppresses the growth of hematopoietic cells transformed by an oncogenic form of STAT5b. Our findings define hTid1 as a novel partner and negative regulator of STAT5b.STAT transcription factors play a central role in cytokinedependent survival, proliferation, and differentiation of a large spectrum of cells. Following cytokine addition, STAT proteins become tyrosine-phosphorylated and subsequently dimerize, forming homo-or heterodimers, and translocate into the nucleus, where they bind to specific elements in the promoter of target genes and activate transcription (1). The STAT protein family comprises seven members, including the two closely related STAT5a and STAT5b molecules (2, 3). Mice in which stat5a and stat5b genes were deleted revealed redundant and specific functions of both proteins. stat5a