Trisomic animals lose third chromosome Generally, when a third sex chromosome is added to the normal two in mammals (XX for female and XY for male), developmental defects result. Mice that are trisomic for the sex chromosomes are infertile. Hirota et al. demonstrate that reprogramming cells from sterile mice with chromosome trisomies XXY or XYY generates XY stem cells. Sperm generated from these XY stem cells could give rise to healthy, fertile offspring. Reprogramming also promoted loss of the extra chromosome in cells from patients with Klinefelter (XXY) or Down (trisomy 21) syndrome. Science , this issue p. 932