The processes and mechanisms that have led to the tremendous species diversity observed today have intrigued people for centuries. Initially, it was believed that the geographical separation of populations formed the most important barrier for reproduction. It is now clear that processes such as sexual selection or environmental changes also contribute to speciation. Despite these advances, the genetic basis of the speciation process itself remains largely unknown. Here we present the GENBAS project, in which the GENomic BAsis of Speciation will be investigated in the cichlid genus Ophthalmotilapia. The genus comprises four species with comparable ecological preferences, and different but partially overlapping distribution ranges in Lake Tanganyika. Their reproductive behaviour, taxonomy and phylogeny are well known, and unidirectional hybridisation between sympatric species pairs has been observed. The males defend a breeding territory, to which females are attracted to spawn. Ophthalmotilapia species are maternal mouthbrooders and females provide care for the fry. The GENBAS project aims (1) to characterize the genomic differentiation that drove the speciation process in Ophthalmotilapia, and (2) to verify whether/and to what extent the same genomic changes are maintaining the integrity of the gene pools of the resulting sister species. To achieve these goals, we will first experimentally assess courtship behaviour and acoustics in the four species, under con- and heterospecific mating trials. Second, gene expression patterns in the female brain during con- and heterospecific courtship behaviour will be assessed through RNAseq. Third, a genome-wide screening of genetic diversity and differentiation among populations of sister species will reveal which genomic regions are under selection in Ophthalmotilapia. Finally, integration of the RNAseq and genome-wide differentiation patterns will indicate whether the genes involved in mate pairing are situated in regions with high or low differentiation which will provide insight in the selective mechanisms driving speciation.