At least five different kinds of changes can occur to a gene:

• Homozygous deletion, where both copies of the gene to be deleted. Such deletions can be quite small - just a single gene - or can stretch for up to 10 million base pairs and delete many genes.
• Subtle sequence change: base pair substitution, or the insertion or deletion of one or two base pairs in the DNA sequence.
• Chromosome breakage or translocation, snapping the gene in two or fusing two genes together.
• Gene amplification, where the number of copies of the gene changes. Some cancer cells can have up to 1000 copies of a particular gene.
• Modifications of the control regions of a gene may alter its activity - turning it on or off inappropriately.