(A) Cultures were inoculated with equivalent numbers of ES cells bearing different Atrx alleles as indicated, and were serially passaged. After the indicated days of coculture, DNA extracted from a sample of cells was analysed by Southern blot to detect the Atrx alleles. DNA was digested with SpeI, and the membrane was hybridised with the 20/27 probe shown in Figure 2A. The expected sizes of the different alleles are indicated.