A 26-year-old man who had childhood cancer has received the first known transplant of sperm-producing stem cells. When Jaiwen Hsu was diagnosed with bone cancer and underwent chemotherapy at age 11, doctors froze some of his testicular cells to improve his chances of becoming a dad in the future. The effort is part of a study at the University of Pittsburgh Medical Center that so far has included about 1,000 boys. “The science behind it is so incredibly new,” Hsu told The Associated Press. “It’s kind of eagerly crossing our fingers and hoping for the best.”