The loss of HBII-52 and related C/D box small nucleolar RNA (snoRNA) expression units have been implicated as a cause for the Prader–Willi syndrome (PWS). We recently found that the C/D box snoRNA HBII-52 changes the alternative splicing of the serotonin receptor 2C pre-mRNA, which is different from the traditional C/D box snoRNA function in non-mRNA methylation. Using bioinformatic predictions and experimental verification, we identified five pre-mRNAs (DPM2, TAF1, RALGPS1, PBRM1 and CRHR1) containing alternative exons that are regulated by MBII-52, the mouse homolog of HBII-52. Analysis of a single member of the MBII-52 cluster of snoRNAs by RNase protection and northern blot analysis shows that the MBII-52 expressing unit generates shorter RNAs that originate from the full-length MBII-52 snoRNA through additional processing steps. These novel RNAs associate with hnRNPs and not with proteins associated with canonical C/D box snoRNAs. Our data indicate that not a traditional C/D box snoRNA MBII-52, but a processed version lacking the snoRNA stem is the predominant MBII-52 RNA missing in PWS. This processed snoRNA functions in alternative splice-site selection. Its substitution could be a therapeutic principle for PWS.