An exon is a fragment of a gene whose DNA sequence, after transcription, is found in the mature mRNA. This part of the gene is often coding.
In order for a protein to be produced, the gene delivers a manufacturing code to the cell. This is composed of "blocks" called exons, which must be assembled end to end. The code is incorrect in genetic diseases (an abnormality is present in one or more exons) and the cell is not able to make the protein. The aim of the exon skip is to remove the coding part containing an error in order to restore the reading framework and enable the cell to synthesise the missing protein.