Complete selective deficiencies of the complement component C1q are rare genetic disorders which are associated with recurrent infections and a high prevalence of lupus erythematosus-like symptoms. The improvements in molecular biology techniques have facilitated the analysis of such genetic defects to a great extend. To date the basis of C1q deficiencies from 13 families have been studied at the genetic level. In each case single base mutations leading to either termination codons, frame shift or amino acid exchanges were thought to be responsible for these defects as no other aberrations were found. In addition to DNA analysis, conventional immunochemical and biochemical methods have contributed substantially to the elucidation of the structural and functional requirements of this complex macromolecule. The present article reviews the different types of C1q defects in regard to structure and function whereas a detailed presentation on the clinical aspects of C1q deficiencies will be given in this issue of the Journal (by WALPORT, DAVIES and BOTTO).