5' splice site mutation. Nucleotide substitutions at positions -1 and +6 of the 5' splice site of the second intron.
An Hsp70 promoter is fused upstream of the w transcription unit in which positions -1 and +6 of the 5' splice site of the second intron have been mutated (G to C and T to C nucleotide substitutions respectively). These mutations are expected to weaken base-pairing of the 5' splice site to the 5' end of U1 RNA.