Abstract: To avoid inbreeding and promote out-crossing, many flowering plants have adopted self-incompatibility (SI) systems, through which incompatible (self or genetically related) pollen is recognized and rejected, whereas compatible (non-self) pollen is allowed to grow in the style to deliver the germ cells to the ovary for double fertilization. Among various SI systems, gametophytic SI in Solanaceae, Scrophulariaceae and Rosaceae appears to be the most common whereby the specificity of SI response is controlled by a single polymorphic S-locus. Recent studies have shown that the S-locus is organized in a haplotype fashion and carries at least two genes determining the recognition specificity: S-ribonucleases expressed in the pistil (pistil-S) and S-locus Fbox(SLF) genes in the pollen (pollen-S). Here we discuss recent data on the possible molecular mechanisms eliciting the S-RNasebased self-incompatibility response.