**5. Conclusions**

The findings reported in the current study further elucidate the prevalence of ASD symptomatology in a sample of individuals with DS, as measured by gold standard diagnostic instrument, the ADOS-2. The findings of this study replicate previously reported findings of increased challenges related to social communication and higher levels of rigid and repetitive behaviors among individuals with DS + ASD in comparison to those with DS-only. We also highlight the contribution of language delays to the classification of ASD in this sample, which underscores previously raised questions regarding the boundary between phenotypic characteristics of DS and true ASD symptomatology. Although this study contributes to the field by examining the prevalence and presentation of ASD in the largest sample of individuals with DS to date, it also demonstrates the need for more research exploring the complexities of diagnosing ASD among individuals with DS.

**Author Contributions:** A.D.N., V.N., C.A., A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; methodology, A.D.N., V.N., C.A., A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; formal analysis, A.D.N., V.N., C.A., L.A., and A.J.T.; investigation, A.D.N., V.N., C.A., A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; data curation, A.D.N., V.N., C.A., A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; writing original draft preparation, A.D.N., V.N., C.A., L.d.H.S., L.A., and A.J.T.; writing—review and editing, A.D.N., V.N., C.A., A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; visualization, A.D.N.; supervision, A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; project administration, A.D.N., V.N., C.A., A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T.; funding acquisition, A.S., J.E., S.S., E.B.-K., L.d.H.S., L.A., and A.J.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the following grant from the National Institutes of Health, R01HD074346, P50HD103526, and UL1TR001860.

**Institutional Review Board Statement:** This study was approved by the Institutional Review Board at all participating university sites. The authors assert that all procedures contributing to this work comply with the ethical standards of the relevant national and international committees on human experimentation and with the Helsinki Declaration of 1975, as revised in 2008 (Code: 403210).

**Informed Consent Statement:** Informed written consent was obtained from the parent/legal guardian prior to participation, and assent was obtained from each participant.

**Data Availability Statement:** The datasets used and/or analyzed for the present paper can be made available upon a reasonable request to the corresponding author.

**Acknowledgments:** The authors thank the many staff across all the participating data collection sites who worked on the project. We are indebted to the families for their time, support, and partnership.

**Conflicts of Interest:** L.A. has received funding from F. Hoffmann-La Roche Ltd., Roche TCRC, Inc., Neuren Pharmaceuticals Ltd., and Lumind to consult on and implement outcome measures in clinical trials for FXS and DS. E.B.-K. has received funding from Seaside Therapeutics, Novartis, Roche, Alcobra, Neuren, Cydan, Fulcrum, GW, Healx, Neurotrope, Marinus, Zynerba, BioMarin, Lumos, Ovid, AMO, Yamo, Ionis, GeneTx, Acadia, Neurogene, Orphazyme, Ultragenyx, Taysha, Tetra, and Vtesse/Sucampo/Mallinkcrodt Pharmaceuticals to consult on trial design or development strategies and/or conduct clinical trials in FXS or other genetic neurodevelopmental or neurodegenerative disorders, and from Asuragen Inc. to develop testing standards for FMR1 testing. A.J.T. has received funding from Fulcrum Therapeutics to develop outcome measures for FXS. The other authors declare that they have no competing interests.
