Lai's lab discovered that the mats gene had a mutation--an additional piece of DNA that had somehow become inserted into the molecular sequence of the gene. This mutation was a type of "transposable element" known to be able to jump to different locations along the DNA. The inserted material disrupted the gene's function, which is to make a particular protein that works in particular ways. "A mutant gene is not going to make the protein product that the normal gene is able to make," Lai explains.
Lai then generated mutant clones of this fly and found that they developed large tumors in many organs including the head, eye, wing, leg, and antenna. He also generated another mutant form of the mats gene--this time with a huge missing pie ce of genetic material caused by another transposable element jumping out and dragging lots of material along with it. "With such a deletion of material, this gene is basically gone, so we knew it couldn't possibly function at all," Lai says. Both kinds of mutations resulted in the formation of the same kinds of tumors. These studies indicate that the disabling of this one gene alone can cause the growth of tumors.
The researchers then did a number of experiments to study this line of flies with the clearly defective mats gene. "With transgenic tec