CIC-5 is a chloride channel known to be expressed in the kidney. We previously reported that CIC-5 mRNA was also strongly expressed in immature human myeloid cell line (HL-60), but weakly expressed in mature neutrophils. To clarify the underlying mechanisms, we examined the relationship between CIC-5 expression and cell cycle. Dimethyl sulfoxide treatment of HL-60 that causes differentiation with G0/G1 cell cycle arrest decreased the expression of CIC-5 rnRNA. Cell sorting and synchronization experiments revealed that CIC-5 rnRNA expression was high in S and G2/M phases and low in G0/G1 phase. CIC-5 antisense oligonucleotide suppressed proliferation of HL-60 cells with a decrease in CIC-5 protein expression, probably due to G2 arrest. These results suggest that cell cycle-dependent expression of CIC-5 has a role in cell cycle progression in myeloid cells. (C) 2004 Elsevier Inc. All rights reserved.