We have proposed a molecular model of susceptibility to Insulin-Dependent Diabetes Mellitus (IDDM) based on the proportional expression at the cell surface of the following susceptible (S-S) HLA-DQ heterodimere composed of a chain DQ alpha Arg 52 positive (S) and a chain HLA-DQ beta Asp 57 negative (S). All other DQ alpha/beta chains associations are considered as protective molecules. This model allows a predictive scale of risk for the disease.