Potential mechanism of the pathogenesis of 20q deletions.

Circles represent genes on 20q. If there is a single target gene on 20q (hatched circle), it may be necessary for both copies to be lost/inactivated (`two-hit' model). Alternatively, loss of a single copy (haplo-insufficiency) may be adequate to produce a phenotypic effect (`one-hit' model). If there is more than one target gene, the `one-hit' model would entail haplo-insufficiency for two or more genes, perhaps scattered over a large distance of the chromosome. In contrast, the `two-hit' model would involve bi-allelic inactivation of at least one of the target genes.