Snyder-Robinson syndrome is caused by a mutation in the spermine synthase (SMS) gene. A gene is a piece of DNA that encodes for a protein that has a function in the organism. Deleterious changes in the SMS gene cause the formation of a protein that does not function properly. Since males have only one copy of the SMS gene, when that copy is not functioning correctly they will be affected. On the other hand, females have two copies of the SMS gene, which is the reason why they are not affected, but can have affected children. The SMS protein is important for many functions of the organism and is normally present in the brain and in the bones, as in many other organ systems.
Mutations in the SMS gene can be detected with sequencing of the gene, which looks at small changes at the nucleotide level, if the doctor suspects this condition. More often, the mutations are found using whole exome sequencing, which looks at the entire coding DNA, or a gene panel that includes the SMS gene among many others.