BLM encodes the protein RecQL3 that repairs aberrant DNA replication.32 Autosomal recessive genotypes in BLM cause Bloom syndrome (#210900), an extremely rare disorder characterized by chromosomal instability, malignancies, short stature, dermatological conditions and reduced immunoglobulins IgM and IgA.32 Of the five recessive genotypes we observe in our Bulgarian cohort, only one involved LOF alleles. Here, CD40LG is linked to Bloom syndrome.