During the initial stages of heat stress, the heat shock transcription factor (HSF) is the primary molecule responsible for relaying signals of cellular stress to the transcriptional apparatus. Metazoan HSFs comprise a rather small gene family consisting of four distinct types.In addition to HSFs that activate transcription, HSF4 in man has no capacity to support activated transcription and has been shown to suppress expression of heat shock genes under control conditions.Based on parsimony analysis of their DNA-binding domains (DBDs) and a comparison of oligomerization domains (ODs), they have been assigned to major classes A and B.These two groups of HSFs appear to be unique to plants with no clear relationships to HSF types in other organisms.