Hemoglobin is isolated from the erythrocytes of a young child with anemia. Hemoglobin electrophoresis reveals the presence of an unstable hemoglobin, known as hemoglobin Cranston (HbCr), containing an abnormal b-globin chain. The normal sequence of the b-globin gene (HbNl) and the sequence of the HbCr b-chain are presented in the table below. HbNl: AAGUAUCACUAAGCUCGC HbCr: AAGAGUAUCACUAAGCUCGCUUUC >>> UAU UAA Which of the following would account for the development of HbCr?