Ras-related nuclear proteins (Rans), a family of small monomeric G proteins, are important in many cell activities, participating in the regulation of eukaryotic nucleocytoplasmic transport, DNA replication, mitotic spindle assembly, nuclear envelope dynamics, etc. In this study, a Cryptocaryon irritans trophonts and characterized. The full-length cDNA was 771 bp, and contained an open reading frame (ORF) of 654 bp, encoding a putative polypeptide of 217 amino acids. The ORF contained 6 TAAs and 2 TAGs, which encode glutamine in the ciliate rather than stop codons, as in the universal genetic code. The calculated molecular weight of the CiRan protein was 25.3 kD, with a pI of 8.9. The sequence information has been deposited in GenBank under accession number KP662712. A bioinformatic analysis predicted that the deduced CiRan protein contains a typical Ran domain, extending from residue 11 to residue 174. A GTP/Mg²⁺-binding site occurs at residues 17-153, a Switch I area at residues 30-50, a Switch II area at residues 67-87, a G1 ring at residues 17-26, a G2 ring at residues 40-42, a G3 ring at residues 65-70, a G4 ring at residues 120-125, and a G5 ring at residues 148-152. The predicted structural characteristics confirm that CiRan is a member of the Ran protein family. A phylogenetic tree of CiRan and 10 Ran proteins from other species revealed that CiRan is closely related to the Ran proteins of Tetrahymena thermophila. Reverse transcription-PCR showed that the C. irritans life cycle. After the non-universal codons, TAA and TAG, in the ORF of the cDNA were modified to CAA and CAG, respectively, and the ORF DNA was amplified and inserted into the I restriction sites of the bacterial expression vector pGEX-4T-1, generating the recombinant plasmid pGEX-4T-1/. The transformed was induced with isopropyl-β-D-1-thiogalactopyranoside to express the recombinant protein rCiRan, as a fusion protein with gluthathione S-transferase (GST). After purification with glutathione Sepharose 4B, rCiRan was used to intraperitoneally immunize specific-pathogen-free Kunming mice to prepare a polyclonal antibody against rCiRan. Another group of mice was immunized with GST to prepare a polyclonal antibody against GST for the negative control. The results of a western blotting analysis showed that the polyclonal antibody against rCiRan recognized the native CiRan protein in lysates of different developmental stages of . The molecular mass of native CiRan was 25.3 kD, which is consistent with the value calculated from the coding sequence. The native CiRan protein in theronts was localized with an indirect immunofluorescent antibody assay with the antibody directed against rCiRan, showing that native CiRan was not only distributed in the cytoplasm of theronts, but also in the nuclei. It was especially abundant around the nuclear membrane, implying that the protein has nuclear-membrane-related functions. This study extends our theoretical understanding of the biology of the pathogen , and lays the foundation for future studies of the functions of CiRan, which may be important in the prevention of infection and the control of cryptocaryonosis.