Haploinsufficiency: A situation in which the total level of a gene product (a particular protein) produced by the cell is about half of the normal level and that is not sufficient to permit the cell to function normally. Another way to define haploinsufficiency is as a condition that arises when the normal phenotype requires the protein product of both alleles, and reduction of 50% of gene function results in an abnormal phenotype.
Haploinsufficiency can be due to a number of problems. One of the two copies of the gene may be missing due to a deletion. A mutation (change) in the gene may have wiped out production of message. Or the message or the protein produced by the cell may be unstable or degraded by the cell.
The term "haploinsufficiency" is of recent vintage. It was cobbled together for use in genetics, cell biology, and molecular medicine from "haploid" (one instead of two chromosomes or two genes) + "insufficiency." Haploinsufficiency is related to hemizygosity.