The Strangers in Your Brain - The New Yorker

If transposons were tampering with the DNA of every future neuron, then they were endowing each one with a slightly different genome. Even neurons that budded from the same mother would behave differently. This phenomenon, which is known as genetic mosaicism, doesn’t happen much in other tissues. The cilia that guard our lungs are genetically identical to the blood cells that circulate in our arteries, even though one looks like a sea anemone and the other looks like a cough drop. The two appear different only because they express various genes differently, in developmentally predetermined ways. Although neurons are similarly programmed, the Salk study suggested that transposons were giving them the ability to ad-lib. Several years after the initial discovery, members of Gage’s lab sequenced hundreds of individual neurons from human cadavers and found this to be true. Cells in the same brain are, indeed, genetically distinct from one another.