The seed storage proteins of Coix, sorghum and maize are codified by homologous genes which are coordinately expressed in the endosperm in a temporal-specific fashion. Opaque2 (O2), a bZIP protein originally isolated from maize, has been described as a transcription activator of alpha- and beta-prolamin genes. The isolation and characterization of cDNA and genomic clones encoding the Opaque2 homologue from Coix are reported here. The coding region of the Coix O2 gene is interrupted by five introns and codifies a polypeptide of 408 amino acids. Comparison of the deduced amino acid sequence with two different sequences of maize O2 protein showed that the Coix O2 protein is similar to the maize O2 isolated from W22 maize inbred line. The Coix O2 protein has the same binding specificity and expression pattern of the maize O2. The O2 proteins together with OHP1, OsBZIPPA, SPA, CPRF2 and RITA1 were assigned to one of the five bZIP plant families in an updated classification of plant bZIP according to bZIP domain similarity.36249-63