Substrates for helicase assays generated by PCR. A PCR product and its restriction fragment derivatives are shown. PCR reactions were performed as described in Materials and Methods with 32 P-labeled primer, and products were purified using the QIAquick PCR purification kit (Qiagen) (lane 1) and digest with PstI (lane 2), EcoRI (lane 3) and SmaI (lane 4) restriction enzymes.