Heat shock induces the synthesis of seven proteins at five otherwise inactive sites in the polytene chromosomes of fruit fly larvae. Two separate sites, producing two and three copies, respectively, code for the 70 kDa protein. Most strains have three copies of the gene coding for this protein at chromosome locus 87C1; two tandemly repeated Hsp70 genes (Hsp70Bb and Hsp70Bc) and one in reverse orientation (Hsp70Ba). Some strains, including that sequenced in the Drosophila genome project have three tandemly repeated Hsp70 genes (Hsp70Bb, Hsp70Bbb and Hsp70Bc). Belongs to the heat shock protein 70 family.