We successfully isolated a rat cDNA clone encoding a novel EF hand protein with a molecular weight of about 17 kDa and designated this gene iba1 (ionized calcium binding adapter molecule 1). The genomic copy of the iba1 gene was located within a segment of the major histocompatibility complex class III region between the Bat2 and TNF alpha genes. The iba1 gene was shown to be highly expressed in testis and spleen, but weakly expressed in brain, lung, and kidney. Among brain cells, the iba1 gene was specifically expressed in microglia. A screening of hemopoietic cell lines showed that the Iba1 protein was clearly expressed in monoblastic cell lines but only very weakly expressed in myeloid cell lines. Iba1 protein is therefore suggested to act as an adapter molecule, mediating calcium signals that may function in a monocytic lineage including microglia.