My comments: Venter and colleagues describe a method for replacing a chromosome in one bacteria by another from another bacteria (the two bacteria being distinct species, although ~95% identical at the genome level in this case). One of the challenges was in the extraction of intact, protein-free chromosomes from cells. The actual transfer then relies on polyethylene glycol (PEG), known to promote fusion of eukaryotic cells, but the actual transplantation mechanism is not understood. The authors hypothesize that two chromosome-free cells might fuse around a naked chromosome, with low frequency (but the resulting cells are selected using an antibiotics resistance gene). Nonetheless, this is clearly an important step forward for synthetic biology (you can imagine that entirely synthesized chromosomes will be inserted into empty host cells using this type of approaches).