Artificial gene synthesis is a method in synthetic biology that is used to create artificial genes in the laboratory. Currently based on solid-phase DNA synthesis, it differs from molecular cloning and polymerase chain reaction (PCR) in that the user does not have to begin with preexisting DNA sequences. Therefore, it is possible to make a completely synthetic double-stranded DNA molecule with no apparent limits on either nucleotide sequence or size. The method has been used to generate functional bacterial or yeast chromosomes containing approximately one million base pairs. Recent research also suggests the possibility of creating novel nucleobase pairs in addition to the two base pairs in nature, which could greatly expand the possibility of expanding the genetic code.