Suzanne de Bruijn

289 General discussion and perspectives Pathogenic variants in other genes, including RHO (1990) 35 , were identified after candidate gene cloning, given the known organ-specific key function of for instance rhodopsin. Also studies in natural mouse mutants were employed successfully, in which a mutated gene was identified and the human counterpart turned out to be mutated in affected individuals. Such studies contributed significantly to an impressive list of candidate disease genes in which often one or two variants were found in a single case or family. 36-38 Efforts undertaken by the International Mouse Phenotyping Consortium (IMPC) are still ongoing to generate and phenotype a knockout mouse model for all genes. 39 1990 1995 2000 2005 2010 2015 2020 300 250 200 150 100 50 0 154 HL genes 271 RD genes Human Genome Project Implementation of WES/WGS Disease-associated genes Year Implementation of LRS Figure 1. Discovery of inherited RD- and HL-associated genes over the years. The first inherited RD- and HL-associated genes were discovered in the early 90s using linkage analysis and positional cloning strategies. Supported by the arrival of novel technologies (next generation sequencing (e.g. whole exome sequencing (WES), whole genome sequencing (WGS) and long-read sequencing (LRS), the number of disease-associated genes gradually increased over the years. Today (May 2021), 271 RD-associated genes (RetNet 31 ) and 154 HL-associated (HHL homepage 32 ) genes have been reported. Although the methods employed in this first era of DNA sequencing are generally considered to be time-consuming and labor-intensive as compared to contemporary parallelized sequencing efforts, they provided crucial insights from which we still benefit today. Most techniques applied in families with inherited RD or HL were focused on limiting the critical region of interest; pinpointing only a small genomic region that