If your child is autistic and has never had genetic testing, a study out of Mount Sinai this week is worth your attention. Researchers at the Seaver Autism Center for Research and Treatment now estimate that Phelan-McDermid syndrome, a condition long filed under "rare," affects roughly 1 in 7,300 people. That works out to more than 45,000 people in the United States. The number of people who carry the diagnosis is a small fraction of that.

That gap between the estimate and the count is where families have something to act on.

What the researchers found

The study, "Prevalence of Phelan McDermid Syndrome Estimated To Be ~1:7300 Using a Multisource Model," was published July 9 in the journal Autism Research. The team analyzed genetic data from nearly 180,000 individuals with autism who had undergone testing, drawing from genetic testing laboratories, academic medical centers, and autism research programs. After accounting for people who were never tested, tests that missed the relevant gene, and individuals with the syndrome who do not meet the criteria for autism, they landed on an estimate of 13.7 cases per 100,000 people.

Earlier counts were far lower. The Phelan-McDermid Syndrome Foundation has recorded somewhere north of 1,200 cases worldwide. An estimate of 45,000 in one country is a different order of magnitude, and it reframes the condition from a medical curiosity into something a general pediatrician might reasonably encounter.

What Phelan-McDermid syndrome is

Phelan-McDermid syndrome is caused by a deletion or mutation of the SHANK3 gene on chromosome 22. SHANK3 helps build the connections between brain cells, so when it is missing or altered, the effects show up across development. The syndrome is associated with intellectual disability, delayed or absent speech, low muscle tone, and a range of medical issues.

Most people with the syndrome also meet the criteria for autism spectrum disorder. Researchers estimate that SHANK3 changes account for up to 1 percent of all autism cases, which is part of why the Seaver team went looking through autism genetic data in the first place.

Why so many cases go unrecorded

Tess Levy, the study's first author and a certified genetic counselor at the Seaver Center, tied the undercount directly to testing access. Many children with developmental disabilities and autism are simply never offered genetic testing. Among those who are, some families run into insurance barriers, and some receive tests that do not adequately examine the SHANK3 gene. A standard chromosomal microarray can miss smaller mutations, and not every ordering physician knows to look.

The result is a large population of children who have a specific, identifiable genetic condition and a diagnosis that reads only "autism" or "developmental delay." For a parent, that distinction shapes which specialists a child sees, which medical screenings make sense, and which research doors are open.

What a diagnosis changes

Joseph Buxbaum, who directs the Seaver Center and served as senior author, put the recommendation plainly: every child with autism should undergo genetic testing, because knowledge is power. He also said he expects successful new treatments for the syndrome within five years.

That second point is the practical engine behind the first. Several precision-medicine trials targeting the underlying biology of Phelan-McDermid syndrome are already underway. Trials recruit by genetic profile, so a child cannot be considered for a SHANK3-targeted study without a confirmed SHANK3 finding. A genetic diagnosis today is what makes participation possible later, and it connects families to a specific patient community and its research registries rather than the broad and diffuse world of general autism support.

A confirmed result also sharpens medical care in the present. Phelan-McDermid syndrome carries known health associations, including kidney abnormalities, seizures, and lymphedema, that warrant monitoring a clinician might not order for an undifferentiated autism diagnosis.

What families can do

If your autistic child has never had genetic testing, ask the child's pediatrician or a developmental specialist about it, and ask specifically whether the panel evaluates the SHANK3 gene. If testing was done years ago, it may be worth revisiting, since sequencing methods have improved and older microarrays can miss the smaller mutations. Families weighing the decision, or making sense of a result, can start with our guide to genetic counseling after a diagnosis and what a rare-disorder finding sets in motion, covered in navigating a rare disorder diagnosis.

Testing does not always return an answer, and a result that names nothing has its own meaning, which we cover in what a negative genetic test means. For the tens of thousands of families the Mount Sinai numbers point to, though, the test is the difference between a label that describes behavior and one that names a cause with treatments coming into view.