<Li> Human embryonic kidney cells (HEK - 293) </Li> <Li> Human embryonic retinal cells (Crucell's Per. C6) </Li> <Li> Human amniocyte cells (Glycotope and CEVEC) </Li> <P> Cell - free production of proteins is performed in vitro using purified RNA polymerase, ribosomes, tRNA and ribonucleotides . These reagents may be produced by extraction from cells or from a cell - based expression system . Due to the low expression levels and high cost of cell - free systems, cell - based systems are more widely used . </P>

What determines the type of protein produced by a cell