The main Rh antigen (the one that determines whether a person is “Rh-positive” or “Rh-negative”) is known to blood bankers as the “D” antigen. A gene known as RHD on chromosome 1 codes for this large transmembrane protein, and it is present in the majority of most people regardless of race. Occasionally, however, mutations in RHD may result in amino acid substitutions in the normal D protein. If those substances change the appearance or structure of exposed parts of the antigen on the outside of the RBC (“epitopes”), the patient is described as having “partial D.” If someone with partial D is given blood from someone with normal D, he may make an antibody against the portions of the antigen that he lacks. The resultant antibody looks just like anti-D, because it reacts against all “normal” D-positive cells. This situation leads to the classic presentation of a partial D patient in the transfusion service as a D-positive patient with anti-D. There are numerous types of partial D, with varying strengths of reactivity with anti-D. The main thing to remember about partial D blood recipients is that they should receive D-negative blood, even though they may appear D-positive. Modern anti-D reagents, while they are very good at detecting weaker forms of the D antigen, are specifically designed to NOT detect the most common form of partial D in Caucasians (DVI, or “D six”), so most Caucasian partial D patients will test as D-negative. Partial D is a “D variant” like “Weak D.” Current recommendations suggest testing D variant blood recipients and pregnant females with molecular testing to further evaluate the risk of receiving D-positive blood in recipients or developing anti-D in D variant pregnant females.