Abstract We have identified a cDNA clone for human cytidine deaminase ( EC 22.214.171.124) during an investigation which aimed at cloning novel gene expression products related to monocyte/macrophage differentiation. The derived amino acid sequence of the clone comprises 145 residues yielding a molecular mass for the polypeptide of 16.1 kDa and exhibits a nearly 50% homology to cytidine deaminase from Bacillus subtilis. Cytidine deaminase activity of the cloned sequence could be demonstrated in a prokaryotic expression system. The mRNA is highly expressed in granulocytes while expression is very low in fibroblasts, chondrocytes, monocytes, and T- as well as B-cell lines. The mRNA can be induced in monocytes, the monocytoid cell line U937 and the myeloblastic line HL 60 by the differentiation inducer calcitriol.