Rh system belongs to the most important blood group systems. Its molecular basis are two highly homologous genes (RHD a RHCE) localized in closed proximity and in opposite orientation on the first chromozome (1p34-36). Products of these genes are two proteins, RhD and RhCcEe, carrying the Rh antigens. RhD protein is the most immunogenic antigen complex of the red cell membrane – RhD negative phenotype is characterized by its total absence – that iswhy the immune system of RhD- people is very sensitive to contact with RhDpositive erythrocytes (frequent antibody production in pregnancy or after transfusion of RhD+ red cells). Other antigenic differences (C+/-, c+/-, E+/-, e+/- aj.) on the other hand are represented by only one or few aminoacid substitutions in one protein and therefore are much less immunogenic. The detection of RhD in cca 1–2% RhD positive samples is complicated by the abnormal expression of the protein, which can be divided into quantitative weakening (less protein copies on the membrane) and into qualitative variants. First type (weak D) is based on RHD(weak) gene mutations which do not affect the extramembranousportion of the protein.The second type (D variants) is based on RHD(variant) gene with alteration(s) resulting in the loss of one or more D epitopes – this feature is used the serologic characterisation of variants and is the cause of their clinical importance (carriers of D-epitope deficient phenotype are able to form antibodies against „missing“ part of the protein when contacted with „normal“ RhD positive red cells). This article is describing one unique RhD variant – RoHar, which dramatically differs from other types due to its molecular background (presence of only one hybrid RHCE-RHD(5)-RHCE gene) as well as serologic characteristics (unusual negative reactions with most IgG anti-D in the antiglobulin test), causing frequent discrepancies and diagnostic complications.

        Key words: Rh system, RHD and RHCE genes, RhD and RhCcEe proteins, weak and variant RhD antigens, D epitopes, R0 Har variant