Possession by a cell of more than one nucleus each from a different parent.

Though it might seem obvious that a heterokaryon is formed upon fusion of and , that is, to produce a zygote, in fact that the resulting cell is too briefly 'heterokaryonic' to normally be described in those terms. Instead, as naturally occurring, the heterokaryotic state is seen particularly in the course of sexual processes as associated with fungi.

Note, also in fungi, that the heterokaryonic state is a consequence of plasmogamy and ends, at least for certain cells, with karyogamy. Thus, lack of heterokaryonic stage → plasmogamy → heterokaryonic stage → karyogamy.

Alternatively as well as additionally, note the concept of dikaryotic such that lack of heterokaryonic stage → plasmogamy → heterokaryonic stage → dikaryotic and/or karyogamy.

Heterokaryons also can be generated .