Proposed evolution of cell lines showing (a) stem line with two normal chromosomes 11 and a dic(12;15), (b) formation of r(11) with dic(12;15) present, (c) two normal chromosomes 11 with chromosome 15 material excised from the dic(12;15) to form a der(12) +/− r(15), (d) two normal chromosomes 11 with an inversion occurring in the dic(12;15) and excision of chromosome 12 material to form a der(15) +/− r(12), (e) r(11) with chromosome 15 material excised from the dic(12;15) to form a der(12) +/− r(15), (f) r(11) with an inversion occurring in the dic(12;15) and excision of chromosome 12 material to form a der(15) +/− r(12), and (g) the formation of the tricentric chromosome der(11;15;12). +/− represents that the cell population is a composite and the ring was observed on at least 2 occasions but was not present in all cells of the population.