With the aid of a dissecting microscope, whole kidneys were manually removed from embryos submerged in phosphate buffered saline (PBS). To dissociate the epithelium, kidneys were subsequently placed in 500 uL of 1% trypsin in Hanks solution. After 90 minutes of incubation at 4ºC, the digestion was stopped with the addition of 1 mL of 20% fetal bovine serum (FBS) in Hanks solution plus a small amount of DNase I powder. Tungsten needles were used to tease away the epithelium which was collected in tubes containing Trizol. Tissue from several litters was collected to obtain enough RNA for SAGE library construction. No more than two litters were dissected on the same day, with each litter taking about 1hour to manually dissocitate after trypsin digestion.