Researchers at Stanford University have bioengineered DNA in living cells so that a portion of that DNA flips under specific environmental conditions. When the DNA is in one direction and is activated, it turns on a gene (in this case the green fluorescent protein), when the DNA is flipped upon replication and it is activated, it turns on another gene (red fluorescent protein). In essence, these researchers have created the genetic equivalent to a binary bit system, depending on the direction of the DNA and the gene it turns on, it is either a one or a zero.
The researchers call this system "recombinase addressable data" module, or RAD for short. For the system to work, they had to control the precise dynamics of two opposing proteins, integrase and excisionase, within the microbes. But once the proper balance was found, they had a reproducible and reliable system. They tested RAD modules in single microbes that have doubled more than 100 times and the switch has held. Furthermore, they switched the latch and watched a cell double 90 times, and then switched in back. In short, it seems like RAD works. It is reliable and it is rewritable.