The loss of one allele at a specific locus, caused by a deletion mutation; or loss of a chromosome from a chromosome pair, resulting in abnormal hemizygosity. It is detected when heterozygous markers for a locus appear monomorphic because one of the ALLELES was deleted. When this occurs at a tumor suppressor gene locus where one of the alleles is already abnormal, it can result in neoplastic transformation (CELL TRANSFORMATION, NEOPLASTIC).
