PEX1 and peroxisome biogenesis disorder: We used CRISPR/Cas9 to introduce a 17-bp deletion in exon 5 of pex1, the zebrafish ortholog of the gene most commonly mutated in ZSD patients, PEX1. The deletion causes a frameshift that inserts a premature stop codon, leading to the absence of Pex1 protein and resulting in a marked reduction of functional peroxisomes.