Since Clusius first documented floral heteromorphy in Primula in the 16th century, the genus has been a model system for those studying the development of different floral morphs on plants of the same species. Over the centuries, eminent botanist and geneticists, including Darwin, Hildebrand and Ernst, have furthered our understanding of the phenomenon in a number of species. In Primula vulgaris (Common Primrose), in which flowers take either a long styled (Pin) or short-styled (Thrum) form, heteromorphy is linked to a sporophytic self-incompatibility system, with both mechanisms under the control of the highly conserved, diallelic Self-Incompatibility (S) locus. Whilst classical genetic approaches have identified basic functions of the S locus, as well as the order of the loci controlling these features within the locus, the molecular structure of this important region of the genome remains unknown. Recently, a number of molecular S-linked markers have been characterised, providing an opportunity to begin molecular characterisation of the locus as well as its immediate surroundings. Using these markers as a guide, a single contiguous sequence has been assembled to join three of these markers together, spanning the region in which the mechanisms preventing recombination within the locus breaks down. Within this region, 51 genes have been identified and annotated. Homologues of these genes have been identified in Solanum lycopersicum, Oryza sativa and Arabidopsis thaliana, providing an insight into the convergence and divergence of genes between the four species.