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Uroporphyrinogen III synthetase (UROS) is an important enzyme involved in the structural formation of porphyrin, which is responsible for color intensity. A UROS homolog was isolated and characterized from the pearl mussel, Hyriopsis cumingii (HcUros). The HcUros cDNA was 858 bp in length with an open reading frame that encoded a 285 amino acid protein, which contained a conserved HemD domain. HcUros exhibited widespread tissue distribution, displaying particular enrichment in the mantle and haemolymph of purple mussels, compared to white mussels. In situ hybridization of HcUros in mantle tissue demonstrated that it was specifically expressed in the dorsal epithelial cells of the mantle pallia. Additionally, HcUros expression was upregulated following implantation of the pearl sac to produce pearls. The data suggest that HcUros contributes to shell pigmentation, and nacre formation and coloring in H. cumingii.