No one has yet demonstrated the perfect gene delivery vehicle. But Chromos Molecular Systems Inc. thinks it has a construct - mammalian artificial chromosomes (MACs) - that possesses many of the desired characteristics.

MACs start with a neutral array of DNA capped at each end by a telomere. Most importantly, the MAC contains a centromere, just as a real chromosome does. Centromeres are regulatory bodies that organize and manage orderly chromosome replication during cell division.