Each year companies that develop new medicines spend millions of pounds on tests that are meant to make sure their new drugs are not only effective, but also safe for us to take. In spite of this, the single biggest problem with new medicines is that sometimes they cause damage to the liver, heart or brain, which results in them being taken off the market. This situation could be greatly improved if the companies involved had ways of testing (at an early stage) the safety of new medicines using cells that more closely match those found in normal human liver, heart and brain. Liver transplantation is an effective treatment for patients suffering from life-threatening liver diseases and disorders but the supply of donor organs is severely limited meaning that many people remain on the waiting list each year. New ways to efficiently make functional liver cells would lead to improvements in both of these areas. Many researchers, including our own groups, have shown that it is possible to turn unspecialised cells called stem cells into liver cells, more specifically cells called hepatocytes. It is the hepatocytes in our livers that are responsible for removing harmful products from our blood throughout our lives. At the moment, the hepatocyte-like cells we can generate from stem cells are not as good as proper liver cells and lack the ability to carry out many liver functions. Also, the methods currently used to turn stem cells into liver cells are very expensive and take approximately 4 weeks. In this proposal we want to develop conditions that allow us to turn stem cells into liver cells more efficiently and cheaply. We also want to test for ways in which we can make hepatocyte-like cells that more closely resemble the normal adult liver cells found in humans. To help us do this we will make stem cells that turn on fluorescent signals (which we call reporters) at particular stages as they progress from stem to liver cells. One of these 'reporters' will switch on part way through the process, after about a week, and will help us to improve the generation of cells formed during the early stages of liver development and we also hope to discover how we can increase the numbers of these precursor cells. We will also make a second reporter stem cell line will switch on only when the cells have developed into mature liver cells. We will use this stem cell line to help us work out those conditions that turn stem cells into hepatocyte-like cells that are as close as possible to proper adult liver cells. We will examine whether the liver cells we generate are like normal adult liver cells by examining their properties. We will determine if they contain proteins only found in liver cells and we will study if they contain the enzymes important for metabolising medicines, since these are very important for proper liver cell function. We hope that our results will define conditions that: (i) improve our ability to turn stem cells into liver cells, (ii) help us to expand the number of liver precursor cells we can make, (iii) enable us to generate hepatocyte-like cells that are more like proper adult liver cells and (iv) improve the efficiency and methods of turning stem cells into liver cells so that they are cheaper to make. The availability of mature hepatocyte-like cells from a renewable source of stem cells will be extremely valuable to companies making new medicines, since it will help them develop drugs that should be safer for patients and the time taken to develop them will be reduced, saving money. Not only would stem cell-derived liver cells useful for testing the safety of new medicines, but in the future they may also be valuable as a source of cells for transplantation. Therefore, the improvements in generating functional hepatocyte-like cells that we discover will also help scientists and doctors who are developing cell transplant strategies and artificial livers as alternative treatments for patients requiring liver transplants.