Stem cells divide to produce daughter cells. These cells then can either form new stem cells or become specialized cells (differentiation) such as brain, muscle, heart, or bone cells. Stem cells can be pluripotent (able to divide into many cell types), unipotent (able to divide into a single cell type), multipotent (able to divide into several cell types), or totipotent (able to divide into all cell types). Embryonic stem cells are totipotent cells derived from 3-5 day old embryos that are in the blastocyst stage. They are extracted from the inner cell mass of the blastocyst or fetal tissue. Adult stem cells typically originate from tissues including umbilical cord tissue, fat, the bone marrow, and blood. When compared in quantity to other cells, there are only a small quantity of stem cells in each tissue. Because of this and their growth rate, it is often difficult to produce adult stem cells in large numbers. iPSCs are a type of pluripotent stem cells which are derived from adult somatic cells (e.g. skin or blood cells). Somatic cells are reprogrammed to induce iPSCs through the expression of transcription factors Sox2, Oct4, c-Myc, and Klf4.