Over the last three decades, advances in sequencing and genotyping techniques have paved the way to linkage analyses (the primary method used in the second half of the 20th century to map genetic loci with familial aggregation [36,37]), which have allowed for the identification of the gene encoding the transcription factor 7-like 2 (TCF7L2), considered to be the most potent locus for T2D risk [38]. The gene discussed is TCF7L2; the disease is type 2 diabetes mellitus.