Like chromosomes, genes can be completely or partly lost or duplicated during cell division. Another piece may also spontaneously join itself to a gene.
At the gene level, a complete or partial abnormality of this kind can sometimes be difficult to trace. This is because many of the standard techniques that are used are unable to distinguish between two identical genes or exons and those that may have been deleted or duplicated.
It is therefore not always obvious that part of the gene has been lost or duplicated.
Sometimes only one or a small number of nucleotides has been lost or duplicated, or one or more nucleotides are added. These are easier to trace. For example we know that a deletion of 3 nucleotides - representing 1 codon - (F508del) causes cystic fibrosis.