A '''loss-of-function''' mutation is a [[mutation]] in the coding sequence of a gene, which causes the function of the gene product to be either reduced or completely absent. Some use the term only when the function is completely absent, and use the related term [[hypomorphic mutation]] when the function is merely reduced (note that the term hypomorphic mutation is also used when the mutation causes reduced ''expression'' of the gene; such a mutation is in a non-coding region and is not considered a loss-of-function mutation).
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A loss-of-function mutation can for instance be caused by the truncation of the gene product because of a [[frameshift]] or [[nonsense mutation]].
Often an organism will function normally with a loss-of-function mutation in an allele, so long as there is at least one functioning allele of the gene in question. If this is the case, then the phenotype associated with the mutated allele will be [[recessive]]. Examples of this are the alleles responsible for [[sickle celle anemia]] and [[cystic fibrosis]]. On the other hand, if the dosage of gene product supplied by the remaining normal alleles is not enough for normal function of the organism (this is called [[haploinsufficiency]]), then the associated phenotype will be [[dominant gene|dominant]].
