The Rh system is the most polymorphic and immunogenic of all blood group systems. Currently, over 49 antigens have been identified, among which the five main antigens are D, C, c, E, e. Knowledge of the Rh system's molecular basis, since its first cloning 17 years ago, has allowed for an understanding of the mechanism of Rh-negative phenotype and of the variants of antigens RHD and RHCE. Deletions, gene rearrangements and insertions are the main mutations. In Caucasians, the primary mechanism of Rh-negative phenotype is the complete RHD gene deletion, whereas in Afrodescendant it is the presence of pseudogene RHDø and hybrid gene RHD-CE (4-7)-D. The authors analyzed the structure of the Rh complex in red cells, the molecular basis of the Rh system, mechanisms of RHD negativity and weak and partial RHD expressions.