Constitutive phosphorylation at Ser303 within the regulatory domain of HSF1 was shown to inhibit its transcriptional activity under normal temperatures. Substitution of Ser303 with alanine derepresses the transactivation domain such that the S303A mutant showed increased transcriptional activity in human and mouse cells (Knauf U et al. 1996; Kline MP & Morimoto RI 1997). Phosphorylation of HSF1 at Ser303 was mediated by glycogen synthase kinase 3 (GSK3) activity in human acute monocytic leukemia THP1 cells and mouse embryonic fibroblast NIH 3T3 cells (Chu B et al. 1996, 1998). However, the other group showed that GSK3 inhibits HSF1 activity in HeLa cells through a mechanism that is independent of Ser303 phosphorylation, thus suggesting that Ser303 may be phosphorylated by multiple MAPKs (Batista?Nascimento L et al. 2011). The phosphorylation at Ser303 in turn promoted HSF1 association with YWHAE (14-3-3 epsilon), which may be involved in the attenuation of HSF1 activity during recovery leading to accelerated cytoplasmic localization of HSF1 (Wang X et al. 2003, 2004).In addition, stress stimulated human K562 erythroleukemia cells showed enhanced level of sumoylation in the HSF1 regulatory domain at Lys298, which was positively regulated by Ser303 phosphorylation (Hietakangas V et al. 2003).