The correction of a cellular defect caused by mutations in a gene by the addition of the normal gene or its product (protein). In cell cultures, complementation can be accomplished by transfer of deoxyribonucleic acid (DNA) or messenger ribonucleic acid (mRNA) that encodes this protein. Another way to “complement” a cellular defect is by joining two chromosome sets in the same cell, each defective due to a different mutation but together able to correct the cellular defect. This principle is used in somatic cell hybridization (or cell fusion), in which cells are fused to generate hybrid cells that possess all chromosomes from both fusion partners. Such hybrids are usually viable and can be examined for the presence of the cellular deficiency via complementation analysis.