Color intensities represent different specificities of csd alleles. The most-variable parts of the allelic sequences (RS domain, proline-rich domain, hypervariable part) are marked in blue and yellow while the more-conserved N-terminal part is shown in green. Model 1: polypeptides that are derived from different alleles associate and form heteromers that are functionally active. The non-functional species is the monomer that is present in haploid and diploid males. Model 2: polypeptides that are derived from the same allele associate and form homomers. Consequently, two homomer species exist in females, while there is only one homomeric species in males. Model 3: no binding but activity differences exist depending on whether polypeptides derive from the same or different alleles. Different complementation groups in different alleles complement each other resulting in a functional protein in heterozygous females. I predict that no differences in binding properties of polypeptides are found whether they are derived from the same or different alleles.