SummaryMutations truncating as many as 143C‐terminal residues from the transcriptional activator encoded by theareAgene, mediating nitrogen metabolite repression inAspergillus nidulans, do not significantly reduce the ability of theareAproduct to activate expression of most genes underareAcontrol. Such mutations can even have a gain‐of‐function, derepressed phenotype, consistent with a critical role for this region in modulating the activity of theareAprotein. However, expression of a few genes underareAcontrol is substantially impaired by suchC‐terminal truncations, indicating that regions of an activator protein can play differing roles in the control of different structural genes. This underlines the advantages of being able to monitor effects ofareAmutations on expression of large numbers of structural genes. Additionally, it is shown that truncation of as many as 153C‐terminal residues, virtually all amino acidsC‐terminal to the DNA‐binding region, is compatible with retention of som