The absence of one copy (allele) of a gene or of part of one copy of a chromosome. In some circumstances, this may imply the presence of a tumour suppressor gene in the lost genetic material.

(synonym: LOH) At a particular locus heterozygous for a deleterious mutant allele and a normal allele, a deletion or other mutational event within the normal allele renders the cell either hemizygous (one deleterious allele and one deleted allele) or homozygous for the deleterious allele Related Terms: deletion ; hemizygous ; heterozygote ; homozygote

In cancer, loss of a second allele of a gene when the first allele is already altered or lost