Treating cancer is difficult because there's no easy way for a treatment to differentiate between healthy cells and cancerous cells. Somehow, you need to know if a cell is cancerous, then treat it, or else don't touch it. It would be great if we could program cells themselves with that logic, and as it turns out, we can.
In a lot of ways, cells are just like little computers. They have a set of instructions that they follow in the form of genes, and these genes can process chemical information and turn themselves off and on, altering the instructions that they communicate to the cell. Since we're now able to design our own artificial genes and introduce them into cells, we can create our own little genetic computers with custom programming, and researchers at MIT and Switzerland's ETH Zurich have been able to construct a genetic logic circuit that tests for cancer and, if it finds it, instructs a cell to kill itself.
Many types of cancer produce little bits of RNA called microRNA in proportions that are distinctly different from healthy cells. Furthermore, different types of cancer produce different kinds of microRNA in different amounts. Taking advantage of this, researchers created a synthetic gene that produces a protein that causes cell death, but in order for the gene to activate, the right types of microRNA in the right amounts have to be present. For example, a cancerous cell might produce a high amount of microRNA that is ordinarily low in healthy cells. The synthetic gene would be programmed to react to high levels of that microRNA, whereas with a low level of the same microRNA, the synthetic gene would stay dormant, keeping the cell death protein from being produced.
The most recent version of this synthetic gene features six different microRNA switches, all of which have to be tripped by just the right mircroRNA pattern in order for the cell death protein to be made. Essentially, the cell is checking six different ways to see whether it has cancer, and if even one of those markers isn't quite right, the cell keeps itself alive.
So instead of having to go figure out which cells are cancerous and which aren't, you can just introduce this genetic logic gate to every cell, and the cells themselves will decide if they're cancerous or not and take action as necessary. The complex logic system ensures that only cancerous cells kill themselves, and since the logic can be tweaked to target different kinds of cancer (or anything else) researchers are optimistic that these genetic circuits might lead to "a general technology for disease-state detection."