Homo sapiens C/D box snoRNA HBII-52-18: this snoRNA was first cloned in the mouse (MBII-52) by Cavaillé et al. (2000), and found to be expressed in all regions of the brain, except for the choroids plexus. HBII-52 is also brain-specific (Cavaillé et al, 2002). Due to cross-hybridisation in Northern blot analysis, it could not be assessed if all predicted snoRNAs in this cluster are indeed expressed. As HBII-85 and HBII-13, HBI-52 was not expressed in the brain of a patient with a paternal 15q11-q13 deletion (Prader-Willi syndrome), but was normally expressed when the deletion was maternally inherited (Angelman syndrome). Genomic sequencing revealed that 48 closely related copies HBII-52 are encoded in introns of the large (460kb), paternally expressed, transcription unit (SNURF-SNRNP-UBE3A AS), that is antisense to the maternally expressed UBE3A gene (Cavaillé et al., 2000, Runte et al., 2001). Among these copies, 6 (# 24, 27, 28, 45, 46 and 47) have mutated C and/or D boxes, and are probably not functional. In the new nomenclature, HBII-52 snoRNAs were numbered 1-48 following their chromosomal order, but entries like HBII-52-24 (SNORD115-24) were left unassigned. Note however that HBII-52-7 and -25 have a non-consensus C box, but were maintained in the list. The HBII-52 snoRNAs are not predicted to guide to 2'O-methylation of an rRNA or snRNA. However, most of them (HBII-52-1, -4 , -5, -6, -8 to -16, -21, -22, -26, -29, -30, -31, -33 to -36, and -38 to -44) present, upstream of the D box, a perfect 17-18 nt complementarity with a portion of the serotonin receptor 5HT-2C mRNA (Cavaillé et al., 2000). The nucleotide predicted to be methylated is known to be subjected to A/I editing. The MBII-52 inhibits the ADAR2-mediated editing of a 5HT-2C minigene transcribed in the nucleolus from a RNA polI promoter (Vitali et al., 2005). Moreover, HBII-52 regulates the alternative splicing of the natural 5HT-2C pre-mRNA (Kishore and Stamm, 2006). The same transcription unit hosts HBII-13, HBII-436, HBII-437 and HBII-438A/B, and 29 copies of HBII-85. Recent studies suggest that the lack of HBII-85, but not HBII-52, is critical in Prader-Willi syndrome (Runte et al., 2005; Ding et al., 2005).