Homo sapiens C/D box snoRNA Z17B. This snoRNA is a close paralogue of mgh18S-121 (Z17A) and is predicted to guide the 2'O-ribose methylation of 18S rRNA U121. This methylation site was mapped by Maden (1990). Its sequence was submitted to GenBank by Zhou,H. and Qu,L.H. (AJ224025). A close paralogue, mgh18S-121 (Z17A), as well as the C/D box snoRNAs U42A and U42B, reside in the same host gene (RPL23A).