AbstractThe epidermal growth factor receptor (EGFR) is important for normal homeostasis in a variety of tissues and, when abnormally expressed or mutated, contributes to the development of many diseases. However, in vivo functional studies are hindered by the lack of adult mice lacking EGFR because of the pre‐ and postnatal lethality of EGFR deficient mice. We generated a conditional allele of Egfr (Egfrtm1Dwt) by flanking exon 3 with loxP sites in order to investigate tissue‐specific functions of this widely expressed receptor tyrosine kinase. The activity of the Egfrtm1Dwt allele is indistinguishable from wildtype Egfr. Conversely, the EgfrΔ allele, generated by Cre‐mediated deletion of exon 3 using the germline EIIa‐Cre transgenic line, functions as a null allele. EgfrΔ/Δ embryos that have complete ablation of EGFR activity and die at mid‐gestation with placental defects identical to those reported for mice homozygous for the Egfrtm1Mag null allele. We also inactivated the Egfrtm1Dwt allele tissue‐specifically in the skin epithelium using the K14‐Cre transgenic line. These mice were viable but exhibited wavy coat hair remarkably similar to mice homozygous for the Egfrwa2 hypomorphic allele or heterozygous for the EgfrWa5 antimorphic allele. These results suggest that the hairless phenotype of Egfr nullizygous mice is not solely due to absence of EGFR in the epithelium, but that EGFR activity is required also in skin stromal cells for normal hair morphogenesis. This new mouse model should have wide utility to inactivate Egfr conditionally for functional analysis of EGFR in adult tissues and disease states. genesis 47:85–92, 2009. © 2008 Wiley‐Liss, Inc.