HBB and beta thalassemia: To seamlessly correct the disease-causing IVII-I G > A splice mutation, we wished to insert a dual selection piggyBac transposon-based excisable cassette (Yusa, 2013) into two different loci located in the vicinity of the disease-causing mutation [Fig. 1(b)] in intron II of the HBB gene in beta-thalassemia iPSCs [Fig. S1(a)] using a double Cas9n approach.