An unstable allele designated o2-m55, was isolated as a derivative of the o2-m5 allele. Whereas the o2-m5 allele is caused by the insertion of one copy of the transposable element Activator (Ac) into the first exon of the maize Opaque2(O2) gene, the o2-m55 allele contains two Ac elements. The position of one copy is identical to the position of the Ac element in the o2-m5 allele. The second copy is present within the first copy, thereby interrupting its structure shortly before the first ATG of the major reading frame of Ac. Both Ac sequences have the same orientation. Excision of the internal Ac element as well as excision of the complete double Ac element was detectable. Truncated double Ac elements comprising the complete internal Ac element and either the proximal or distal fragment of the interrupted Ac element can also be excised. The Ac elements exhibit a strong negative dosage effect: kernels which display revertant sectors in a mutant background are rarely seen in plants homozygous for the o2-m55 allele. If only one dose of the o2-m55 allele is present in endosperm tissue, revertant sectors can be detected. The amount of the transcript expressed from the internal Ac element of the o2-m55 allele is less than that derived from the single Ac element present in the o2-m5 allele.