The coding exon sequences of a gene must be assembled at the RNA level, i.e. the intron sequences must be removed. This process is called "splicing". The splicing apparatus recognizes the beginning and the end of an exon, the so-called exon-intron boundary, by the base sequence in this region. If an essential base of this recognition sequence is exchanged, the corresponding exon is not considered in the splicing process, resulting in the synthesis of an altered protein.