So this is called building a complementary DNA library. So a complementary cDNA library is a library that consists of eukaryotic genes in which we have removed all these different introns. So to see how we can build a cDNA library, let's take a look at the following five steps. So let's suppose we take this same eukaryotic DNA molecule that contains these introns and these exons. So this is our DNA library shown here. So we have these introns, we have the exons and this is our gene, the eukaryotic gene.