Relative expression levels of representative placental markers at E15.5. (A) Junctional zone markers. Achaete-scute complex homolog 2 (Ascl2) is required for spongiotrophoblast differentiation (Guillemot et al., 1994). Trophoblast specific protein-α (Tpbpa) is expressed in spongiotrophoblast and glycogen cells (Lescisin et al., 1988). Heart and neural crest derivatives expressed transcript 1 (Hand1) expressed in spongiotrophoblast cells and giant cells, including sinusoidal trophoblast giant cells (Riley et al., 1998; Simmons et al., 2008a). Prolactin family 8, subfamily a, member 8 (Prl8a8) and Prl3a1 are spongiotrophoblast-cell-specific markers (Simmons et al., 2008a). FMS-like tyrosine kinase 1 (Flt1) is expressed in spongiotrophoblast cells, and fetal and maternal endothelial cells (Breier et al., 1995). *P=0.014. (B) Labyrinth zone markers with schematic illustrating the organisation of the trilaminar trophoblast layer with gene expression markers. Dlx3 is expressed broadly in the labyrinth without indication of layer-specificity at E14.5 (Simmons et al., 2008a). Kinase insert domain protein receptor (Kdr; also known as Flk1) is expressed in fetal endothelial cells with a key role in vascular development (Shalaby et al., 1995). Glial cell missing 1 (Gcm1) encodes a transcription factor required for the initiation of fetal blood vessel branching (Anson-Cartwright et al., 2000) and is expressed specifically in syncytiotrophoblast II cell layer by E14.5 (Simmons et al., 2008a). Pleckstrin homology-like domain, family A, member 2 (Phlda2) is expressed in syncytiotrophoblast layers I and II (Qian et al., 1997; Frank et al., 1999; Dunwoodie and Beddington, 2002). Syna is expressed in syncytiotrophoblast layer I (Simmons et al., 2008a). Placental lactogen II (Prl3b1) is expressed in both the parietal giant cell layer and sinusoidal trophoblast cells of the labyrinth (Jackson et al., 1986). Cathepsin Q (Ctsq) is expressed exclusively in the sinusoidal trophoblast giant cells (Simmons et al., 2007). Solute carrier family 2, member A1 (Slc22a1) is expressed both in the junctional zone and the labyrinth zone, and Sc22a3 expression is restricted to the sinusoidal trophoblast cells (Shin et al., 1997). *P=0.014. Data expressed as relative fold change in comparison with wild type. n=4+4. Error bars show standard deviation. (C) In situ hybridisation with Prl3b1 riboprobe of wild-type and Cdkn1c mutant placental sections at E15.5. Lower magnification images (4×) illustrate diminished staining in regions containing spongiotrophoblast cells and parietal and sinusoidal giant trophoblast cells, all of which are cell types that express this marker. Higher magnification (10×) specifically illustrates Prl3b1 staining of sinusoidal giant trophoblast cells in wild-type and mutant labyrinth.