Heat shock transcription factors (HSFs) function to regulate the expression of heatshock proteins (hsps) or molecular chaperones in the cell. Mammalian cells have twowell-characterized HSFs, HSF1 and HSF2. HSF1 functions to regulate the stress-inducedexpression of hsps. The function of HSF2 appears to be in regulating hsp expressionduring development and differentiation.In this work, I describe two distinct HSF1 mRNA isoforms (HSF1-a and HSF1-b)that are generated by alternative splicing of the HSF1 pre-mRNA. The two HSF1 mRNAisoforms result from the inclusion (HSF1-a), or omission (HSF1-b), of a 66 nucleotideexon of the HSF1 gene, which encodes a 22 amino acid sequence. These results showthat the levels of the HSF1-a and HSF1-b mRNA isoforms are regulated in a tissuedependentmanner, with testis expressing predominantly the HSF1-b isoform while heartand brain express primarily the HSF1-a isoform.In addition, I describe two distinct HSF2 mRNA isoforms (HSF2-a and HSF2-b)that are generated by alternative splicing of the HSF2 pre-mRNA. The two HSF2 mRNAisoforms result from the inclusion (HSF2-a), or omission (HSF2-b), of a 54 nucleotideexon of the HSF2 gene, which encodes a 18 amino acid sequence. These results showthat the levels of the HSF2-a and HSF2-b mRNA isoforms are regulated in a tissuedependentmanner, with testis and brain expressing predominantly the HSF2-a isoformwhile heart, liver, and kidney express primarily the HSF2-b isoform. Furthermore, HSF2isoform levels are regulated both in a developmental and cell type dependent manner inthe testis. In a reporter assay, HSF2-a is a 2.6-fold better transcriptional activator thanthe HSF2-b isoform.We have demonstrated also that HSF2, but not HSF1 is a substrate for SUMO-1and SUMO-2 modification in vitro. Consistent with this, we have demonstrated thatHSF2 can interact with a portion of Ubc9, the SUMO-1 conjugating enzyme, in a twohybridassay. We have also shown that GFP-HSF2 colocalizes with SUMO-1 in discretenuclear domain structures in 7% of GFP-HSF2 expressing HeLa cells. Finally, we haveshown that lysine 82 of HSF2 is the primary site of SUMO-1 modification in vitro