The HRDC (Helicase and RNase D C-terminal) domain has a putative role in nucleic acid binding. Mutations in the HRDC domain associated with the human BLM gene result in Bloom Syndrome (BS), an autosomal recessive disorder characterised by proportionate pre- and postnatal growth deficiency; sun-sensitive, telangiectatic, hypo- and hyperpigmented skin; predisposition to malignancy; and chromosomal instability [<cite idref="PUB00005473"/>].