The basement membrane is a sheet-like extracellular matrix that underlies epithelia and surrounds muscles. In the gut of
Drosophila, the stiff basement membrane surrounds the muscles used in peristalsis to keep them flat and smooth. Collagen IV is one of the main components of the basement membrane, where it adds structure and stiffness. The
Peroxidasin (
Pxn) gene encodes an enzyme that crosslinks collagen IV at the NC1 domain. This crosslinking supports basement membrane stability and contributes to its stiffness. Hypomorphic P-element mutations of
Pxn exist in
Drosophila that survive to the larval stages with decreased viability to adulthood, but no null mutation has been reported. We expected a null mutation to be lethal at the end of embryogenesis, when collagen IV mutants die. Using CRISPR, we created a mutant (
Pxn11) that deletes a portion of the catalytic domain, eliminating its activity. Further, this deletion also caused a frameshift mutation that inserted a stop codon soon after the deletion. Contrary to our expectations, about twenty percent of expected homozygotes survive and live an apparently normal lifespan. These homozygotes exhibit muscle defects in the gut consistent with loss of stiffness in the basement membrane. A few hypotheses that could account for their viability will be discussed.