We propose that the CD1 gene was originally generated by tandem duplication of MHC class I/II precursor genes in the MHC, followed by sub-functionalization. Subsequently, part of the MHC region was translocated and differentially silenced, leaving MHC class I/II genes in the MHC, with CD1 in the translocated MHC region (MHCtrans). This MHCtrans was later translocated into another chromosome, coincidentally an MHCpara (shown here on human chromosome 1). Dotted boxes indicate silenced/pseudogenes. 2R: second round of whole genome duplication. See text for further discussion.