The observed perinatal lethality indicates that Ptdsr plays an important role during development. Analysis by RT-PCR (data not shown) showed that Ptdsr is expressed early in development, because we were able to detect Ptdsr transcripts in ES cells and embryos at all developmental stages. To analyze in more detail the temporal and spatial expression patterns of Ptdsr, and to correlate expression patterns with observed pathological malformations, we made use of a Ptdsr-β-geo gene-trap reporter mouse line generated from a Ptdsr gene-trap ES cell clone. This line has an insertion of β-galactosidase in the 3' region of the gene (Figure 2a).