AbstractActive Activator (Ac) elements undergo mutations to become nonautonomous Dissociation (Ds) elements at a low frequency. To understand the mechanism of Ds formation, we have developed high-throughput genetic and molecular screens to identify these rare Ds derivatives generated from any Ac insertion in the maize genome. Using these methods we have identified 15 new Ds elements derived from Ac insertions at eight different loci. Approximately half of the Ds elements contain filler DNA inserted at the deletion junction that is derived from sequences within or adjacent to Ac. In contrast to previous reports, several of these Ds elements lack direct repeats flanking the deletion junctions and filler DNA in the donor Ac. To accommodate our findings and those of others, we propose a model of slip mispairing during error-prone repair synthesis to explain the formation of state II Ds elements in maize. We discuss the use of these lines and molecular techniques developed here to capture somatic Ds transposition events in two-component Ac/Ds tagging programs in maize.