Genes for MOB-I, YOU-1, and YOU-2 extended-spectrum beta-lactamases have been cloned and sequenced. The gene for MGH-1 has the sequence of bla(TEM-10), YOU-2 has that of bla(TEM-12), and YOU-1 has that of bla(TEM-26). All have evolved from bla(TEM-1b) but have the strong dual promoter sequence of bla(TEM-2).