We next assessed the functional consequence of the NDD/trigonocephaly associated PJA1 p.R376C variant in vivo by generating a knock‐in mouse line (Pja1KI/Y) with a c.1093C>T (p.R365C) variant, which is equivalent to the p.R376C variant in human, and a knockout mouse line (Pja1KO/Y) with a Pja1 frameshift truncation variant, c.729_744GGAACCGGTGGTGAGAdel (p.E243fs) (Fig. 2A). This evidence concerns the gene PJA1 and trigonocephaly.