Promotes polyploidy in dark-grown seedlings. Regulates the endocycle leading to hypocotyl elongation. Specifically expressed in immature siliques. Specifically expressed in mitotically dividing cells of seedlings. Contains a PAM2-like motif, which seems to be involved in the binding to the PABC/CTC domain of PAB proteins. Overexpression of CID5/IPD1 leads to increased ploidy levels and longer hypocotyls in dark-grown seedlings compared to wild-type.