at the transcriptional level. Analysis of the Drosophila HSP70
gene and comparison of di¡erent heat shock regulatory regions
led to the identi¢cation of a palindromic `heat shock
element' (HSE), CnnGAAnnTTCnnG [4^6]. More recent results
suggested the view that HSEs are composed of contiguous
arrays of a variable number of the highly conserved sequence
nGAAn arranged in alternating orientation [7^9].
These elements bind the trans-acting heat shock factors
(HSF) which are normally present in the cytoplasm in a
monomeric form (with no DNA-binding activity) and which,
upon heat stress, form trimers and migrate to the nucleus to
bind HSEs with high a¤nity [10^15].