sentence: If the PuroR cassette and the 3 ′ homologous arm had successfully integrated into the hESC genome by homologous recombination , the primer sets PTCH1-3 ′ - F ( P3 , in the PuroR cassette ) and PTCH1-3 ′ - R ( P4 , in the PTCH1 locus , downstream of the 3 ′ homologous arm ) would amplify a 1.4-kb product or would not generate a DNA product .