Fig. 1. Generation of hEGFR^KI/KI mice. (A) Schematic representation of the targeting strategy employed to insert a floxed human EGFR cDNA (hEGFR) into the mouse Egfr locus. The full-length hEGFR expression cassette (grey box) contains its own ATG and polyA (pA) site and is flanked by loxP sites (grey triangles). After homologous recombination, the hEGFR cDNA will be inserted into the first exon of the mouse Egfr gene (black box) and placed under the control of the endogenous mouse Egfr promoter. This correctly targeted allele will be referred to as the hEGFR knock-in allele (hEGFR^KI). The neomycin resistance gene (RSV-neo) and the thymidine kinase gene (HSV-tk) are shown. The broken lines delineate the homology regions in the targeting vector, the horizontal bar indicates the Southern blot probe, and the black arrowheads indicate the PCR primers employed for genotype analysis. X, XbaI; BX, BstXI; N, NdeI. (B) PCR analysis of genomic DNA isolated from the progeny of hEGFR^KI/+ intercrosses. (C) Photograph of 6-week-old hEGFR^KI/KI and littermate control mice showing that hEGFR^KI/KI mice are smaller and display short fur hair. (D) Weight representative of a hEGFR^KI/KI and control littermate mouse at different postnatal ages. At all stages, hEGFR^KI/KI mice are significantly smaller than their littermates. w, weeks; m, months