Production of female mice is undesirable ... from parental ES cell lines from one of two genetic backgrounds: B6 albino (C57BL/6J-Tyrc-2J) or 129 (129S1/SvImJ and 129P2/OlaHsd).

During a routine examination of our department's animal husbandry unit, we found a female Swiss albino mouse with an extremely distended abdomen (Fig. 1). This 6-month-old mouse, which was ...