Because the mutation creates an additional splice acceptor site (the 3'-end) upstream of the normal acceptor site of intron 1, the 19 nucleotides that are usually found at the 3'-end of the excised intron 1 lariat can remain behind as pa of exon 2. Exon 2 can, therefore, have these extra 19 nucleotides at its 5'-end. The presence of these extra nucleotides in the coding region of the mutant mRNA molecule will prevent the ribosome from translating the message into a normal b-globin protein molecule. Those mRNA for which the normal splice site is used to remove the first intron will be normal, and their translation will produce normal b-globin protein.Ref: Lippincott illustrated review, Sixth edition