Fig. 1. Organization of the third chromosome chorion gene cluster and amplification regulatory elements. The chorion genes are indicated by arrows. Stimulatory regions (`amplification enhancing regions' or AERs) are indicated by hatched boxes. The ACE3 and ori-ß elements, which are necessary and sufficient for amplification, are indicated by black boxes. Evolutionarily conserved sequences within ACE3, regions of homology between ACE3 and ori-ß, and the location of the and ß sequence elements are indicated. All numbering is relative to the published sequence for the 3.8 kb SalI fragment of the third chromosome chorion gene locus (Levine and Spradling, 1985).