I tried to get two fragments 3.0 kb and 1.6 kb from PCR using genomic DNA as template. But I only got 1.6 kb, and in the same gel the 3.0 kb smear. I used DMSO and Betaine, and I am sure that genomic DNA, primers, dNTP, KOD, and ddH2O are new, clean and work well. Other factors like temperature, cycles, concentration of MgSO4 are OK according to the instructions from Invitrogen. I ned your help, thanks a lot!