Haploinsufficiency occurs when the amount of a gene product, typically a specific protein, produced by a cell is about half of what is considered normal, leading to impaired cellular function. This condition also describes instances where the expression of a normal trait requires the protein products from both gene alleles, and reducing gene function by 50% leads to an atypical phenotype.Several issues can cause haploinsufficiency. One of the gene's two copies might be missing due to a deletion, a mutation could prevent the gene from producing its message, or the resulting message or protein might be unstable or degraded by the cell.The term "haploinsufficiency" is relatively new, created for use in fields like cell biology and molecular medicine. It combines "haploid" (referring to one chromosome or gene instead of two) and "insufficiency." Haploinsufficiency is also related to the concept of hemizygosity.