Using expression cloning techniques, a protein was isolated which gave rise to an outwardly rectifying chloride current after expression in Xenopus laevis oocytes [Paulmichl et al., Nature 1992;356:238-241]. We termed this protein ICln and found it to be crucial for the regulatory volume decrease after swelling cells in hypotonic medium. Here we describe the cloning of the zebra fish homologue to ICln and, in addition, specify the epitope recognized by the antibodies we raised in rabbits against a synthetic peptide comprised by the C-terminal 24 amino acids of ICln·