Homo sapiens U91 snoRNA. This RNA was cloned from HeLa cells by Darzacq et al. (2002) and is the orthologue of MBII-119 cloned by Hüttenhofer et al. (2001). It is predicted to guide the 2'O-ribose methylation of U4 snRNA C8. By FISH experiments, U91 was localized in Cajal bodies. It was later characterized as mgU4-8 by Tycowski et al. (2004), and localized in the nucleolar fraction. mgU4-8 is a part of a larger guide RNA, mgU12-22/U4-8 (see this entry), that resides in the nucleoplasmic fraction, in agreement with a localization in Cajal bodies. It is therefore possible that the localisation of U91 in Cajal bodies by FISH was attributable to the hybridisation of the probe with its precursor mgU12-22/U4-8. After the overexpression of U91 in HeLa cells, FISH experiments revealed a nucleolar and Cajal body localisation, suggesting that the mature U91 indeed resides in the nucleolus (P. Richard and T. Kiss, personal communication).