The NAC domain is widely distributed in plants but so far, has not been found in other eukaryotes. Genetic analysis of mutant alleles and expression assays proved that NAC domain proteins comprise at least one component of the SAM specification network. Recent studies support the conjecture that NAC genes function at the transcriptional level.The arabidopsis genome contains more than 90 predicted NAC domain genes.Most AtNAC genes contain three exons with the last exon encoding an activation domain. A subfamily of AtNAC genes contain additional terminal exons that may encode protein domains that expand the range of biological functions of the proteins encoded by the NAC domain superfamily.