KNOX class 1 homeobox genes play a crucial role in formation and/or maintenance of the shoot apical meristem (SAM) in plants, and their SAM-specific expression is essential for normal development of plants. We examined regulation of expression of OSH1, a KNOX gene in rice. A 5′ upstream region of OSH1 had the ability to direct the expression of a reporter gene in leaf in addition to the SAM. Introduction of OSH1 cDNA without a promoter sequence caused ectopic expression of both endogenous OSH1 and introduced OSH1 cDNA itself into the leaf, which resulted in morphological abnormalities in the leaf resembling those of the overexpressors. These results indicate that an extra copy of OSH1 exons with no promoter sequence disrupts suppression of OSH1 in the transgenic leaf. Introduction of cDNAs of two other KNOX genes, OSH15 and OSH71, also showed ectopic expression. These results suggest that the exon sequences of KNOX genes function as one of the major cis-regulatory elements of KNOX gene expression. We present and discuss a possible model which interprets these results.