Moreover, reading out data encoded in DNA’s four-letter alphabet is straightforward and relatively fast these days with DNA sequencing machines. The problem is writing the data, which typically requires synthesizing custom strands of DNA one letter at a time. Today’s fastest DNA writers can synthesize about 320 million bytes of DNA data per day. At this speed, writing a single gram’s worth of DNA would take nearly 2 million years. “It’s unaffordable compared to hard drives because the writing speed is quite slow,” says Long Qian, a computational biologist at Peking University.