For study inclusion, probands were required to meet DSM-IV criteria for OCD 23 with onset of obsessions and/or compulsions before the age of 18 years (mean = 9.4 years; SD=6.35). Subjects disease, schizophrenia, severe mental retardation that does not permit an evaluation to characterize the psychiatric disorder, Tourette disorder (TS), or OCD occurring exclusively in the context of depression (secondary OCD) were excluded. In addition, individuals were removed from the sample if they were previously diagnosed with brain pathology including brain tumors, Huntington’s Disease, Parkinson’s Disease, or Alzheimer’s Disease. Each case was evaluated by a PhD-level clinical psychologist using the Structured Clinical Interview for DSM-IV (SCID) modified and extended to include additional symptom and diagnostic information as indicated in the supplementary material. Final diagnostic status was assigned based on the consensus of two psychiatrists or psychologists reviewing the case independently. Both parents of the proband were also recruited whenever possible. When parents were unavailable for participation, unaffected siblings were recruited. Genotyping was performed at the Johns Hopkins SNP Center using Illumina’s HumanOmniExpress bead chips (Illumina Inc., San Diego, CA, USA). More details on the diagnostic assessment and the genotyping process are provided in the supplementary materials and methods.
