Homo sapiens C/D box snoRNA mgh18S-121. This snoRNA was discovered in the laboratory of Tamas Kiss (Toulouse) by the examination of conserved intronic sequences in the RPL23A gene, the host gene for the C/D box snoRNAs U42A and U42B. Its expression is not been experimentally verified. Its sequence was submitted to GenBank by Zhou,H. and Qu,L.H. (AJ224024). This snoRNA is predicted to guide the 2'O-ribose methylation of 18S rRNA U121. This methylation site was mapped by Maden (1990). A close paralogue, Z17B (AJ224025), as well as the C/D box snoRNAs U42A and U42B, reside in the same host gene (RPL23A).