Introns occupy about 40% on average of the total length of genes, which means that most randomly occurring mutations will fall into intron regions, and do not affect protein sequences and functions.

What happens if there is a mutation in an intron?

Mutations in these sequences may lead to retention of large segments of intronic DNA by the mRNA, or to entire exons being spliced out of the mRNA. These changes could result in production of a nonfunctional protein. An intron is separated from its exon by means of the splice site.

How would a mutation in an intron affect a protein?

This mutation is located within the intron 19 and creates a novel donor site that results in the inclusion of an 84-bp cryptic exon into the mature mRNA. This cryptic exon contains an in-frame STOP codon, and thus, the translated protein is shorter and nonfunctional (Sanz et al. 2017).