Mermaid was identified in an ENU mutagenesis screen. A G-to-A mutation was identified at the first base of intron 22 (GRCm39:chr8:124634494C>T), changing exon/intron 22 splice donor T-GT to T-AT, which results in exon 21 splicing directly to exon 23. This results in a frameshift generating a premature stop codon. Crossing to a genetrap allele of Nup133 failed to complement the phenotype, (J:137199)