The proteins which carry the Rh antigens are transmembrane proteins, whose structure suggest that they are ion channels.[12] The main antigens are D, C, E, c and e, which are encoded by two adjacent gene loci, the RHD gene which encodes the RhD protein with the D antigen (and variants)[13] and the RHCE gene which encodes the RhCE protein with the C, E, c and e antigens (and variants).[14] There is no d antigen. Lowercase "d" indicates the absence of the D antigen (the gene is usually deleted or otherwise nonfunctional).