The presence in the same cell of two or more genetically different nuclei. Heterokaryosis occurs naturally in certain fungi, in which it results from the fusion of the cytoplasm of cells from different strains without the fusion of their nuclei. The cell, and the hypha or mycelium containing it, is known as a heterokaryon; the most common type of heterokaryon is a dikaryon. Heterokaryosis can also be induced in vitro, to study the interaction between the cellular components from different species (see cell fusion).