Schematic diagram of the RCCX module. Each chromosome 6 can carry one, two, three, or (rarely) four consecutive modules. The RP gene of the first module is always the RP1 allele; if additional RCCX modules are present, they are initiated by RP2. Similarly, the final RCCX module ends with TNXB, whereas the others carry TNXA. Components of the RCCX modules are not drawn to scale.