Seeds are the primary source of nutrition for humans and livestock. Early in human civilization, certain plants provided a sufficient yield of seed to assure an adequate food supply within a small geographical area, and this led to the development of communal living. The work of Mertz and Nelson launched a succession of genetic studies aimed at improving the protein quality of cereal seeds. In the 1960s and 1970s there was much optimism for improving seed protein quality; many predicted that the use of high-lysine cereals would soon curb protein deficiencies in humans. To investigate the potential of genetic engineering for improving the nutritional quality of seed storage proteins, attempts have been made to modify storage protein genes from maize, soybean, French bean, and Arabidopsis. Nearly all of the molecular genetic studies aimed at improving the protein quality of seeds have been conducted with model plant systems.