Haploinsufficiency occurs when a single functional copy of a gene is not enough to maintain normal function. This can happen when one copy of a gene is mutated or deleted, leading to a reduced level of the gene's product, often a protein. In such cases, the remaining functional copy cannot compensate for the loss, resulting in various health issues or developmental problems. This genetic phenomenon is significant in understanding certain diseases, including some forms of cancer and genetic disorders. For example, p53, a tumor suppressor gene, exhibits haploinsufficiency, where losing one copy increases the risk of tumor formation.