A web-based compendium of molecular alterations in primary immunodeficiency diseases. RAPID hosts information on sequence variations and expression at the mRNA and protein levels of all genes reported to be involved in primary immunodeficiency disease (PID) patients. The main objective of this database is to provide detailed information pertaining to genes and proteins involved in primary immunodeficiency diseases along with other relevant information about protein-protein interactions, mouse studies and microarray gene-expression profiles in various organs and cells of the immune system. RAPID also hosts a tool, mutation viewer, to predict deleterious and novel mutations and also to obtain mutation-based 3D structures for PID genes. Thus, information contained in this database should help physicians and other biomedical investigators to further investigate the role of these molecules in PID.