em (ES) cells for gene targeting [33], thus generating a Ptdsr-null allele in a pure, isogenic C57BL/6J genetic background. The newly established knockout mouse line was named Ptdsrtm1Gbf (hereafter referred to as Ptdsr -/-). Heterozygous Ptdsr+/- mice were viable and fertile and showed no obvious abnormalities. Ptdsr +/- mice were intercrossed to ge