Human thymocyte nuclear protein 1 contains a unique DUF55 domain consisting of 167 residues (55-221), but its cellular function remains unclear. Crystals of DUF55 belonged to the trigonal space group P3(1), but twinning caused the data to approach apparent 622 symmetry. Two data sets were collected to 2.3 A resolution. Statistical analysis confirmed that both data sets were partially twinned by tetartohedry. Tetartohedral twin fractions were estimated. After the structure had been determined, only one twofold axis of rotational pseudosymmetry was found in the crystal structure. Using the DALI program, a YTH domain, which is a potential RNA-binding domain from human YTH-domain-containing protein 2, was identified as having the most similar three-dimensional fold to that of DUF55. It is thus implied that DUF55 might be a potential RNA-related domain.