Cell culture is essential for growth of viruses because they are obligate intracellular parasites; they cannot replicate in any cell-free medium, and they require living cells from a suitable host within which to multiply. Animals, such as mice and embryonated avian eggs, may be used for the propagation of viruses, but for various reasons (time, cost, uniformity, ease of handling, animal welfare, etc.), the propagation of most viruses in cultured living cells is the method of choice today. Cell culture is also used in recombinant DNA technology and many in vitro assays. In fact, most of what researchers know about the function and regulation of mammalian genes comes from studying genes transfected into cell culture.