Haploinsufficiency

Haploinsufficiency occurs when a cell is incapable of producing the total level of a gene product (typically a protein). It happens only in diploid organisms, when only one of the gene copies is functional, while the other copy carries a mutation (e.g. a deletion in the gene itself).

In other words, if the normal phenotype requires protein production of both alleles and one is not functioning, there will only exist about 50% of the quantity necessary for the proper functioning of the cell, resulting in an abnormal phenotype, leading to disease (some cancers).