sentence: If the 5 ′ homologous arm and the loxP-flanked inverted puromycin-resistant cassette ( PuroR ) had successfully integrated into the hESC genome by homologous recombination , the primer sets PTCH1-5 ′ - F ( P1 , in the PTCH1 locus , upstream of the 5 ′ homologous arm ) and PTCH1-5 ′ - R ( P2 , in the PuroR cassette ) would amplify a 1.5-kb product or would not generate a DNA product .