Mice heterozygous for this targeted mutation are viable and fertile. Homozygotes die from respiratory failure shortly after birth. Expression of the "knocked-in" <i>lacZ</i> gene is detected in a pattern similar to the endogenous transcript (in lung, skeleton, stomach, duodenum, spleen, and pancreas), and serves as a faithful reporter for Hedgehog (Hh) signaling. Because Hh and fibroblast growth factor (Fgf) signaling are abnormal, homozygous mice have defects in many Hh target tissues. Although the primary lung buds form in homozygotes, lung branching morphogenesis is defect beginning at 10.5 days postcoitus (dpc), and fails to generate a complete respiratory tree. The single left lobe also is significantly reduced, and total lung size at birth is diminished 67-75% compared to wildtype. In addition, homozygous mice have delayed mineralization of the endochondral skeleton, as well as impaired pancreas morphogenesis, islet formation and endocrine cell proliferation. These mutant mice may be useful in studies of Hh and Fgf signaling, organ/tissue development, and cancer.

Loss of <i>Hhip1</i> function in these mice results in specific defects in two Hedgehog target issues, the lung, a target of Sonic hedgehog (Shh) signaling, and the endochondral skeleton, a target of Indian hedgehog (Ihh) signaling.

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