Three common arrangements of the RCCX module. Monomodular, bimodular, and trimodular are present in about 17, 69 and 14% of chromosomes, respectively . The C4 gene which is either C4A or C4B can be either a long or short variant depending upon the presence of a 6.36 kb endogenous retrovirus, HERV-K(C4). Pseudogenes or gene fragments are red. CYP21A1* indicates either a CYP21A1P pseudogene or the CYP21A2 gene. In the present study 40 chromosomes from autistic individuals had a C4B null allele. Of these chromosomes 19 were monomodular and 21 were bimodular. In the control subjects with C4B null alleles 10 chromosomes were monomodular and 1 was bimodular. Bimodular C4B null alleles were significantly more frequent in autistic subjects compared to controls (P = 0.0001). No C4B null alleles or CYP21A2 mutations were detected in the subjects (2 autistic, 4 control) with trimodular RCCX modules, determined by protein immunofixation electrophoresis .