A change or "spelling mistake" in the DNA of a gene that can cause a disease (but sometimes does not cause disease).

A mutation is any change in the normal order of DNA "letters" (bases), a "spelling mistake" in the DNA, that define a specific gene. A disease-causing mutation is one which either disrupts the process of making mRNA (see mRNA) or alters the activity of the protein coded by that gene sufficient to make it non-functional. For example, many different mutations in the rhodopsin gene may cause retinitis pigmentosa (RP).