Embryonic stem cell research is the utilization of stem cells from embryos which can then be grown in a laboratory culture and produce specialized cells to treat diseases or used for research purposes. The most common source of such cells is discarded embryos at fertility clinics - the process of IVF typically produces two or three 'leftover' embryos for each treatment, which are usually discarded. The use of embryonic stem cells in medicine has met with some controversy, particularly from the pro-life movement that is concerned about the destruction of embryos that they believe constitute human life.
Adult stem cell research is similar except it does not use destroyed embryos, and instead is based on stem cells that can be taken from adults. These cells are less versatile than those harvested from discarded embryos, but they do not raise the associated ethical dilemmas  and so far have been easier to work with in achieving results.
The primary reason for wanting to use embryonic stem cells is that they are capable of producing any type of tissue in the body. Thus, theoretically, embryonic stem cell treatments offer far greater potential than adult cells - while both are capable of repairing some damage, embryonic cells could go so far as to replace entire organs. Conversely though, the greater flexibility of embryonic cells also makes them more difficult to control. A major obstacle to their use in humans is a tendency to form tumors.[Citation Needed] Much research is being carried out to better understand the cellular processes which cause this.
- President Bush's refusal of federal funding for new embryonic stem cell lines didn't halt major stem-cell advances, any more than the prohibition against life-threatening research on human subjects, such as the infamous Tuskegee Study of Untreated Syphilis in the Negro Male, stopped the advance of medical treatments. Trading Places - Will the secular left soon attack the religious right for being pro-science? - Joseph Bottum
Expectation and Potential
Jeremy Pearce of the New York Times wrote:
- Dr. Ira B. Black, chairman of the neurosciences and cell biology department of the Robert Wood Johnson Medical School, at the University of Medicine and Dentistry of New Jersey, described the potential of all stem-cell research as threefold. He said the expectation in the laboratory was for such cells to revive damaged and dead cells; to act as vehicle cells in introducing gene therapy; and, finally, to rally and harness the human body's own existing stem cells.
- The end goal would be to replace brain cells lost to Alzheimer's, repair injured nerve cells causing paralysis and treat cancers, malfunctioning organs and other now-irreparable conditions.
- To Dr. Black, the use of embryonic cells fundamentally constitutes the gold standard in medical research because of their purity and versatility. But he cautioned: This is a very young field. Our areas of ignorance are far greater than our areas of knowledge. 
Inducible Pluripotent Stem Cells
Research in 2006 has facilitated the conversion of human adult somatic cells into pluripotent stem cells, namely possessing an ability to differentiate into many types of tissue. (Unlike embryonic stem cells from blastocysts, however, IPSCs are not totipotent.) By forcing the expression of certain genes, via the activation of many transcription factors (such as tetramer and hexamer TFs), using a particular medium (e.g. HIF and LIF), pluripotent stem cells can be generated. This research has great potential on account of its ability to resolve the dilemma surrounding the use of embryonic stem cells.