A gene dosage effect that occurs when a diploid requires both functional copies of a gene for a wild-type phenotype. An organism that is heterozygous for a haploinsufficient locus does not have a wild-type phenotype.
Loss of one copy (one allele) of a gene is sufficient to give rise to disease. Haploinsufficiency implies that no dominant-negative effect of the mutated gene product has to be invoked.
Situation where one normal copy of a gene alone is not sufficient to maintain normal function. It is observed as a dominant mutation on one allele (or deletion of it) resulting in total loss-of-function in a diploid cell because of the insufficient amount of the wild-type protein encoded by the normal allele on the other haplotype (see also dominant negative). A recent example of haploinsufficiency by Kurotaki et al, 2002 in Sotos syndrome.