A cell line is a permanently established cell culture that will proliferate indefinitely given appropriate fresh medium and space. Cell lines have revolutionized scientific research and are being used in vaccine production, testing drug metabolism and cytotoxicity, antibody production, study of gene function, generation of artificial tissues (e.g., artificial skin) and synthesis of biological compounds e.g., therapeutic proteins. Immortalized cell lines are derived from a variety of sources that have chromosomal abnormalities or mutations that permit them to continually divide, such as tumors. Because immortalized cells continuously divide, they eventually fill up the dish or flask in which they are growing. Cells isolated directly from human or animal tissue using enzymatic or mechanical methods. The adherent cells are usually derived from tissues of organs. Suspension cells do not require attachment for growth and are said to be anchorage-independent cells.