them safer for insertion. Then, the desired genetic material is inserted and ligated to the vector. In the case with tumor suppressor genes, genetic material which encodes p53 has been used successfully, which after application, has shown reduction in tumor growth or proliferation. Non-viral methods The non-viral method of transferring genetic material is used less often than the viral method. However, the non-viral method is a more cost-effective, safer, available method of gene delivery not to mention that non-viral methods have shown to induce fewer host immune responses and possess no restrictions on size or length of the transferable genetic material. Non-viral gene therapy uses either chemical or physical methods to introduce genetic material to the desired cells. The chemical methods are used primarily for tumor suppressor gene introduction and are divided into two categories which are naked plasmid or liposome-coated plasmids. The naked plasmid strategy has garnered interest because of its easy to use methods. Direct injection into the muscles allows for the plasmid to be taken up into the cell of possible tumors where the genetic material of the plasmid can be incorporated into the genetic material of the tumor cells and revert any previous damage done to tumor suppressor genes. The liposome-coated plasmid method has recently also been of interest since they produce relatively low host immune response and are efficient with cellular targeting. The positively charged capsule in which the genetic material is packaged helps with electrostatic attraction to the negatively charged membranes of the cells as well as the negatively charged DNA of the tumor cells. In this way, non-viral methods of gene therapy are highly effective in restoring tumor suppressor gene function to tumor cells that have either partially or entirely lost this function. Limitations The viral and non-viral gene therapies mentioned above are commonly used but each has some limitations which must be considered. The most important limitation these methods have is the efficacy at which the adenoviral and adeno-associated vectors, naked plasmids, or liposome-coated plasmids are taken in by the host’s tumor cells. If proper uptake by the host’s tumor cells is not achieved, re-insertion introduces problems such as the host’s immune system recognizing these vectors or plasmids and destroying them which impairs the overall effectiveness of the gene therapy treatment further. Examples Retinoblastoma protein (pRb). pRb was the first tumor-suppressor protein discovered in human retinoblastoma; however, recent evidence has also implicated