For northern blots, either total RNA (30 μg) was extracted from embryos, electrophoresed and transferred to a nylon membrane (Hybond N; Amersham) or a polyA+ RNA northern blot (OriGene Technologies Inc., Rockville, USA) was hybridized using as the probe a Ptdsr fragment amplified from wild-type cDNA using forward primer 5'-GTTCCAGCTCGTCAGACTCG-3' and reverse primer 5'-TGCCCCTAAGACATGACCAC-3'.