The function of C1D (C1D nuclear receptor corepressor, Ensembl gene identifier ENSG00000197223) is as follows. Plays a role in the recruitment of the RNA exosome complex to pre-rRNA to mediate the 3'-5' end processing of the 5.8S rRNA; this function may include MPHOSPH6. Can activate PRKDC not only in the presence of linear DNA but also in the presence of supercoiled DNA. Can induce apoptosis in a p53/TP53 dependent manner. May regulate the TRAX/TSN complex formation. Potentiates transcriptional repression by NR1D1 and THRB (By similarity). {ECO:0000250|UniProtKB:O35473, ECO:0000269|PubMed:10362552, ECO:0000269|PubMed:11801738, ECO:0000269|PubMed:17412707, ECO:0000269|PubMed:9679063}.