by gel electrophoresis. Before the DNA fragments are transferred to a solid membrane which is either nylon or nitrocellulose membrane it is first denatured by alkaline treatment. After the DNA fragments are immobilized on the membrane, prehybridization methods are used to reduce non-specific probe binding then the fragments on the membrane are hybridized with either radiolabeled or nonradioactive labeled DNA, RNA, or oligonucleotide probes that are complementary to the target DNA sequence then detection methods are used to visualize the target DNA. DNA Isolation: The DNA to be studied is isolated from various tissues. The most suitable source of DNA is known as blood tissue. However, it can be isolated from different tissues (hair, semen, saliva, etc.). DNA digestion: Restriction endonucleases are used to cut high-molecular-weight DNA strands into smaller fragments. This is done by adding the desired amount of DNA which can be changed according to the probe used and the intricacy of the DNA, with the restriction enzyme, enzyme buffer and purified water then incubate the reaction at 37 °C overnight. Gel electrophoresis: The DNA fragments are then electrophoresed on an agarose gel to separate them by size. If some of the DNA fragments are larger than 15 kb, then before blotting, the gel may be treated with an acid, such as dilute HCl. This depurinates the DNA fragments, breaking the DNA into smaller pieces, thereby allowing more efficient transfer from the gel to membrane. Denaturation: If alkaline transfer methods are used, the DNA gel is placed into an alkaline solution (typically containing sodium hydroxide) to denature the double-stranded DNA. The denaturation in an alkaline environment may improve binding of the negatively charged thymine residues of DNA to a positively charged amino groups of membrane, separating it into single DNA strands for later hybridization to the probe (see below), and destroys any residual RNA that may still be present in the DNA. The choice of alkaline over neutral transfer methods, however, is often empirical and may result in equivalent results. Blotting: A sheet of nitrocellulose (or, alternatively, nylon) membrane is placed on top of (or below, depending on the direction of the transfer) the gel. Pressure is applied constantly to the gel (either using suction, or by placing a stack of paper towels and a weight on top of the membrane and gel), to ensure good and even contact between gel and membrane. If transferring by suction, 20X