Final answer:
A base substitution at a 5' splice junction is most likely attributable to a shorter than usual final transcript.
Step-by-step explanation:
A base substitution at a 5' splice junction is most likely attributable to a shorter than usual final transcript. When there is a base substitution at the 5' splice junction, it can disrupt the normal splicing process, leading to the exclusion of an exon and resulting in a shorter mRNA transcript.