Fig. 1. Generation of Tbx20 null mice. (A) Schematic representation of the wild-type Tbx20 locus (exons I-IV indicted as black boxes) above the Tbx20 targeting construct and targeted Tbx20^Flox and Tbx20^lacZ alleles. Arrows represent the direction of transcription of neomycin (neo) and lacZ genes. Neo is driven by the phosphoglycerokinase promoter. Arrowheads represent loxP sites. P1 and P2 indicate 5' and 3' Southern screening probes. (B,C) Validation of the primary targeting event in embryonic stem cells using Southern analysis of ScaI- and NcoI-digested DNA and probes P1 and P2. (D) PCR genotyping assay detecting wild-type (174 bp), Floxed (220 bp) and/or Cre-deleted (302 bp) Tbx20 alleles in mice or embryos of indicated genotypes. (E) RT-PCR analysis of RNA extracted from wild-type or Tbx20^lacZ/lacZ embryonic hearts demonstrating deletion of exons I-III and preservation of exons IV-VI (read-through from lacZ cassette) in the mutant. (F-I) Expression of lacZ in Tbx20^lacZ/+ embryos at E7.5-10.5. am, amniotic mesoderm; cp, cardiac progenitors; da, dorsal aorta; e, eye primordium; ecc, endocardial cushion; en, endothelium; h, heart; hb, hindbrain; ias, inter-atrial septum; la, left atrium; N, NcoI; neo, neomycin; nt, neural tube; pe, pharyngeal endoderm; ra, right atrium; S, ScaI; v, ventricle; wt, wild type; ysm, yolk sac mesoderm.