Fig. 1. Absence of forelimb skeletal elements in newborn Tbx5^lox/lox;Prx1Cre pups. (A) Tbx5^lox/lox;Prx1Cre pup viewed from the side. The forelimb has failed to form although the hindlimb has developed normally. (B) Ventral view of the pup shown in A. The arrow indicates a small flap of skin, present on one side of the embryo. (C) Tbx5 expression in the developing forelimb but not the developing hindlimb of an E10.5 mouse embryo. (D) PCR analysis identifies the presence of the wild-type (wt) and conditional allele (lox) in heterozygous Tbx5^lox/wt embryos, the conditional allele (lox) in the homozygous Tbx5^lox/lox embryos and the lox-out deleted allele in Tbx5^lox/lox;Prx1Cre limb buds. (E) Skeletal preparation of a control littermate. (F) Dorsal view of the thoracic region of the embryo in E showing the pectoral girdle and forelimb skeletal elements. (G) An outline of the skeletal preparation shown in F. The forelimb skeletal elements are highlighted in red. (H) Skeletal preparation of a Tbx5^lox/lox;Prx1Cre pup. (I) Dorsal view of the thoracic region of the embryo shown in H. All the elements of the forelimb and pectoral girdle are absent. (J) An outline diagram of the skeleton shown in I. FL, forelimb; HL, hindlimb.