Secretion mutants that block protein exit from the endoplasmic reticulum define genes involved in the formation, targeting and fusion of a small vesicle intermediate. SEC genes corresponding to the mutants defective in vesicle budding define the cytoplasmic machinery responsible for transport vesicle morphogenesis. A biochemical reaction that reproduces ER vesicle budding was reconstituted with gently-broken yeast cells and pure recombinant Sec proteins required in vivo for this budding...