A lot of today's engineers got their start as kids playing with electronics kits, wiring together resistors or capacitors. Depending on how you put the pieces together, you could build a simple switch or make bulbs blink like a movie marquee.

Scientists are now starting to tinker in a similar way with a new sort of circuitry. Instead of electronic components, they're wiring together genes, building "circuits" and injecting them into living bacteria. In a typical genetic circuit a certain chemical might trigger one gene to act as a toggle switch, turning on another gene that makes a bacterium glow. A second chemical might then trigger a third gene, which flips the first one off. Scientists can even program cell colonies to grow into circles or hearts in the petri dish.

Just as electronics kits hint at the principles behind more sophisticated electronics, these simple demonstrations presage a new kind of bio-technology. Scientists may soon be able to program cells--including human cells--just as engineers now program computers and robots.

Scientists are already inserting genes into bacteria to make them produce precursors to insulin. But that doesn't begin to exploit the sophisticated inner lives of bacteria. Using a huge number of proteins, bacteria routinely process signals from the environment and use them to make decisions about, say, whether to produce a toxin or swim toward light. This circuitry of life has been discovered only recently; now engineers and physicists have joined forces with biologists to harness it.

The first programmable cells may serve as sensors that detect biowarfare agents or produce drugs that aren't possible with genetically engineered bacteria. Earlier this year, for example, James Collins of Boston University and his colleagues reported turning bacteria into ultraviolet-light sensors. They inserted one genetic switch into the bacteria that turned on after a pulse of UV light, and a second that made the bacteria form a film on the test-tube walls.

Scientists are most excited about the prospect of programming human cells. Stem cells may ultimately be taught to build bones or livers. Or it may be possible to make gene therapy far more precise by injecting entire gene circuits into human cells. Patients might swallow a pill that carries an "on" signal; if they react badly, they could simply swallow an "off" pill. Hacking into our own genetic machinery sounds like science fiction, but one day it may seem like another kind of tinkering.