The 4q21 microdeletion syndrome is a newly described syndrome associated with facial dysmorphism, progressive growth restriction, severe intellectual deficit and absent or severely delayed speech. The disease is chromosome 4q21 deletion syndrome (Monarch Disease Ontology id MONDO_0013292). Also known as: 4q21 microdeletion syndrome, Del(4)(q21), chromosome 4q21 deletion syndrome, isolated cases, monosomy 4q21.