Fig. 1. Targeted inactivation of the Ltbp1L gene. (A) Schematic representation of the Ltbp1 short (S) and long (L) forms. (B) Targeting of the Ltbp1L locus. The Ltbp1L gene is composed of 34 exons (represented as vertical bars above the gene). Four exons specific for Ltbp1L, and not for Ltbp1S, are presented as red bars. Two null alleles, designated A and B, were generated. Deletion `A' encompasses 457 bp of exon 1 and a portion of intron 1. Deletion `B' is 7.8 kb long and includes most of exon 2 and a portion of intron 2. (C,D) Validation of Ltbp1L gene targeting. (C) Upper panels: northern blot with E14.5 wild-type and knockout (KO) RNA (left) and wild-type and KO RNA isolated from immortalized E17.5 MEFs (right), hybridized to a [³²P]-labeled Ltbp1L cDNA. Lower panels: RNA loading control. (D) Western blots with total-protein extracts isolated from wild-type and KO E14.5 hearts incubated with an Ltbp1 antibody (upper panels) and a ß-actin antibody (lower panels). (E) Newborn control and Ltbp1L^-/- mice. Ltbp1L^-/- neonates are cyanotic and unable to suckle (arrow illustrates the lack of milk in the stomach).