Prokaryotic and eukaryotic cells respond to thermal and chemical stress by inducing a group of genes collectively designated heat shock genes. In eukaryotes, this gene expression is regulated primarily at the transcription level. Heat shock transcription factors 1 and 2 (HSF1 and HSF2), also designated HSTF1 and HSTF2, are involved in this regulation and are upregulated by estrogen at both the mRNA and protein level. HSF1 is normally found as a monomer, whose transcriptional activity is repressed by constitutive phosphorylation. Upon activation, HSF1 forms trimers, gains DNA binding activity and is translocated to the nucleus. HSBP1 (heat shock factor-binding protein 1), also known as HSF1BP or NPC-A-13 (nasopharyngeal carcinoma-associated antigen 13), is a 76 amino acid nuclear protein that binds HSF1 and acts as a negative regulator of the heat shock response.