Reed B. Wic1mer Section on Genetics of Simple Eukaryotes Laboratory of Biochemical Pharmacology National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases Bethesda, Maryland 20892 While most genes are chromosomal, the nonchromosomal genes have played a disproportionate role in molecular biology, in part because of their easy accessibility and in part because they represent the most mobile portion of a cell's genome. This is particularly evident in prokaryotes, where plas- mids and phages have been the central objects of study. The lower eukaryotes, including fungi and...