Nazneen Rahman and colleagues show that biallelic, loss-of-function mutations in CEP57 cause a constitutional mosaic aneuploidy syndrome. These findings show that CEP57 function is crucial for maintaining correct chromosomal number during cell division. Using exome sequencing and a variant prioritization strategy that focuses on loss-of-function variants, we identified biallelic, loss-of-function CEP57 mutations as a cause of constitutional mosaic aneuploidies. CEP57 is a centrosomal protein and is involved in nucleating and stabilizing microtubules. Our findings indicate that these and/or additional functions of CEP57 are crucial for maintaining correct chromosomal number during cell division.