Total RNA from MEM cells or from individual embryos was extracted using the Illustra RNAspin kit (GE Healthcare Europe, Barcelona, Spain). Embryos showing major malformations were not used for mRNA determinations to avoid artefacts arising directly from embryonic dysmorphogenesis16. Determinations were carried out in the Core Facilities of the Genomics Unit at the Scientific Park of Madrid (http://fpcm.es). Quantitative PCR for Alx3 was performed with TaqMan Assay-on-Demand primers and the Taqman Universal PCR Master Mix, No AmpErase UNG (Applied Biosystems, Alcobendas, Madrid, Spain). In other cases, SYBR Green detection was used with Power SYBR Green PCR Master Mix (Applied Biosystems) and the primers indicated in Supplementary Table S2. PCR reactions were performed in triplicate and values were normalized to Gapdh mRNA levels using the double delta Ct method. Stability of Gapdh as a reference for normalization was confirmed using geNorm68, and results are shown in Supplementary Fig. S4.