IntroductionLeukocyte development requires the coordination of stage-specific transcription factors to help orchestrate the processes by which a progenitor cell emerges as a functional leukocyte. Indeed, aberrant expression or function of many of these transcription factors has been associated with several disease conditions, such as leukemia, lymphoma, autoimmunity, and chronic inflammation. Moreover, recent studies have indicated that Krüppel-like factors (KLFs) may be among those key trans-acting factors contributing to the orchestration of several aspects of leukocyte biology, including cell lineage commitment, differentiation, and function.The original Krüppel factor was characterized in Drosophila melanogaster as a "gap" segmentation gene, homozygous mutation of which resulted in the absence of thorax and anterior abdomen in embryos. [1][2][3][4] Thus, the German researchers named this gene Krüppel (English "cripple"). A conserved family of nuclear proteins encoded by Drosophila Krüppel were identified in 1986 and exhibited a striking structural similarity to the DNA-binding "finger motif " of transcription factor IIIA. 5 The first mammalian gene with homology to Krüppel was identified in 1993, and its encoded protein was named erythroid Krüppel-like factor (EKLF) in accordance with its erythroid cell-specific expression. 6 The function of EKLF was demonstrated by the fact that EKLF bound to human and murine adult -globin CACCC elements via its DNA-binding domain, whereas the non-DNA-binding domain mediated transcriptional activation. 7 The importance of EKLF in differentiation and development was later demonstrated by loss-offunction studies showing that homozygous EKLF Ϫ/Ϫ mice developed a fatal -thalassemia during fetal liver erythropoiesis. 8,9 To date, members of the mammalian KLF family number 17. 10 Identified by various experimental approaches, KLF1 (EKLF) through KLF17 have been termed according to their chronologic order of identification ( Figure 1). Each family member is a zinc finger transcription factor. The distinguishing feature of KLFs compared with other zinc finger-containing proteins, therefore, is the presence of a highly conserved DNA-binding domain composed of 3 C 2 H 2 zinc fingers at or near the C-terminus. [11][12][13] As such, most KLFs are able to bind the CACCC element and GC box consensus sequences. Furthermore, the KLFs share a highly conserved 7-residue sequence, TGEKP(Y/F)X, between zinc fingers. 14 The non-DNA-binding regions of each, however, are highly divergent and can function as trans-activation or trans-repression domains. Collectively, these features distinguish the KLFs from the larger family of zinc-finger transcription factors ( Figure 1A).By regulating gene transcription, KLFs are involved in many physiologic and pathologic processes, such as cell differentiation, proliferation, cell growth, and apoptosis during normal development or under different disease conditions ( Figure 1B). 13,15,16 This review focuses on the transcriptional control of leukocyte cell b...