Laser cell-sorter of sperm. Blue beam of laser light illuminates the nozzle (top centre) of a fluorescence activated cell-sorter of human sperm. Male" and "female" sperm can thus be sorted. The sperm are treated with a fluorescent marker that attaches in proportion to the amount of DNA present in each sperm head. "Male" (Y) chromosome sperm which produce a male foetus have less DNA than "female" (X) chromosome sperm which produce a female foetus. The X sperm thus glow brighter in the cell-sorter; 1/4 million sperm can be sorted per hour. In vitro fertilisation can then produce a male or female foetus. Using this technique many sex-linked genetic diseases could be prevented."