One traditional gene-therapy method relies on homologous recombination. The desired gene segment is placed on a small plasmid and delivered by virus or liposome, but this approach has variable expression levels and instability, and is limited by a small insert size. Yeast artificial chromosomes circumvent some of these problems, but YACs cannot be propagated in mammalian cells. Now a team of scientists at DNAVEC Research of Ibaraki, Japan, has come up with a solution: They have developed a method to create artificial mammalian chromosomes.

Chromosomes require three elements, the authors write in US patent 6,716,608: telomeres, a centromere, and an origin of replication. Mammalian telomeric structure is simple enough, consisting of repeated units of the TTAGGG sequence. Centromeres and structures of replication origin, though, are more enigmatic.

To get around this problem, the team inserted human genomic DNA into YACs and looked for so-called alphoid sequences, which are found in...

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