Abstract
A step-wise method for cloning intron-containing genes from genomic DNA is described. The two exons of the human proinsulin gene were separately amplified in two steps using, in the first step, completely homologous primers. This reduces unwanted interactions between mismatched primers and a complex DNA template such as genomic DNA. The fragments were amplified in a second step polymerase chain reaction (PCR) using mismatched primers that incorporated additional bases complementary to the other exon, and these products were spliced together in a third step PCR.