The human β-globin wild-type allele and a certain mutant allele are identical in sequence except for a single base-pair substitution that changes one nucleotide at the end of intron 2. The wild-type and mutant sequences of the affected portion of pre-mRNA are

                   Intron 2                        Exon 3        _
wild type     5′-CCUCCCACAG      CUCCUG-3′
mutant.       5′-CCUCCCACUG      CUCCUG-3′

Speculate about the way in which this base substitution causes mutation of β-globin protein.