There are 10 - 40 times as many proteins as there are genes. The sequence of amino-acids cannot always be predicted from the sequence of DNA as many genes code for more than one polypeptide. One mechanism is the alternative splicing by spliceosomes. Introns can be removed from pre-mRNA molecules in a variety of ways to give different proteins or variants of the same protein.Alternatively, protein modification could lead to the shortening or lengthening of a protein. Amino-acids could be substituted or modified e.g. by citrullination. Scientists are seeking to create new biological pathways and synthesise new genes by genetic engineering. Synthetic biology is an emerging field in science which saw in 2010 an entire artificial bacterial genome synthesised.