Sly didn’t like Louis. It wasn’t just his manner but his appearance that bothered her on some really deep level. He was a trisomy 11. Thanks to some freak accident of cell division, either his father’s sperm or his mother’s egg had carried two copies of chromosome 11, not the usual one, which meant that the zygote—the fertilized egg— that would eventually become Louis the decker had three copies of chromosome 11, not the normal two. Trisomy was a rare but well-studied genetic abnormality that carried with it a certain suite of physical impairments: retarded growth, a weak cardiovascular system, limited motor coordination, and a phenotype or appearance that reminded Sly uncomfortably of an anthropomorphic slug. It also led to mental impairment. The brain of a trisomy was generally unable to attend to the important features of the environment, unable to extract the "signal"—what the person wanted to concentrate on—from the background "noise"—everything else impinging on the senses. Usually this led to the arresting of the victim’s mental development at a level not much higher than a newborn baby’s.