The Shrunken-2 (Sh2) and Brittle-2 (Bt2) genes of maize encode subunits of the tetrameric maize endosperm ADPglucose pyrophosphorylase. However, in all sh2 and bt2 mutants so far examined, measurable ADPglucose pyrophosphorylase activity remains. We have investigated the origin of the residual activity found in various sh2 and bt2 mutants as well as tissue specific expression and post-translational modification of the Sh2 and Bt2 proteins. Sh2 and Bt2 cDNAs were expressed in Escherichia coli and antibodies were prepared against the resulting proteins SH2 and BT2 specific antibodies were used to demonstrate that SH2 and BT2 are endosperm specific, are altered or missing in various sh2 or bt2 mutants, and have a mol. wt. of 54 and 51 kDa respectively in the wild type. The Sh2 and Bt2 transcripts are also endosperm specific. Ten sh2 and eight bt2 mutants show varying severity of phenotypes expressed at transcript, protein subunit and kernel level. Synthesis of multiple transcripts and proteins commonly occurs as a result of sh2 or bt2 mutation. While all mutants produce detectable enzymic activity, not all produce detectable transcripts and proteins. To examine the origin of the apparent non-SH2/BT2 endosperm enzymic activity, homologs of Sh2 and Bt2, designated Agp1 and Agp2 respectively, were isolated from an embryo cDNA library and found to hybridize to endosperm transcripts distinct from those of Sh2 and Bt2. Thus Agp1 and Agp2 or closely related genes may be responsible for the residual activity in some sh2 and bt2 mutants. Surprisingly, no evidence of post-translational modification of the SH2 and BT2 protein subunits was detected.