Human genetic engineering may provide humanity with the ability to build “custom children” and cure multiple hereditary diseases. This can be achieved by modifying a human's genotype to produce the desired phenotype. The result could cure both birth defects and hereditary diseases such as cancer and AIDS. Human genetic engineering can also allow humanity to permanently remove a mutated gene through embryonic screening and allow parents to choose desired traits for their children. Adverse outcomes of this technology may include the transmission of harmful diseases and the production of genetic mutations. The benefits of human genetic engineering outweigh the risks by providing humanity with cures for multiple deadly diseases. Human genetic engineering has the ability to transmit usually fatal diseases. Although transmission is highly unlikely, it is one of the risk factors that scientists have taken into great consideration. If animal cells or organs are transplanted into humans, zoonotic diseases can spread. Bovine spongiform encephalopathy, porcine endogenous retroviruses, and Nipah encephalitis are all potentially fatal zoonotic diseases that could be transferred (Glenn). According to Linda MacDonald Glenn, JD, LLM, “The introduction of these diseases into the human population could have devastating consequences” (Glenn). Human genetic engineering can also cause the production of unwanted mutations such as developmental problems. Procedures that would be used to genetically modify human cells would include numerous alterations to sperm, eggs, stem cells or embryos before entering a woman's uterus. This could potentially change the growth and development of the fetus in ways that have not yet been observed. “Cancer treatment with genetically modified T cells.” National Center for Biotechnology Information. PubMed Central (PMC), June 12, 2011. Web. December 4, 2013. http://www.ncbi.nlm.nih.gov/pubmed/21663987Pray, Leslie A., Ph.D. "Embryo Screening and the ethics of human genetic engineering". Nature.com. Nature Publishing Group, 2008. Web. 1 December 2013. http://www.nature.com/scitable/topicpage/embryo-screening-and-the-ethics-of-human-60561Seck, Chris. “Arguing for and against genetic engineering.” The Stanford RSS Magazine. Np, 08 June 2007. Web. 4 December 2013. http://stanfordreview.org/article/arguing-and-against-genetic-engineering/