Hoarding is characterized by difficulty discarding or parting withpossessions, resulting in clutter that impedes the normal use ofliving spaces [Frost and Hartl, 1996]. Although hoarding can be asymptom of multiple neurological and psychiatric disorders, suchasobsessive-compulsivedisorder(OCD),mountingevidencesug-geststhatonceotherprimarycausesareruledout,hoardingcanalsobe a discrete disorder [Mataix-Cols et al., 2010]. The DSM-Vtaskforce is currently considering the creation of a new diagnosticcategory named hoarding disorder (www.dsm5.org). We haverecently shown that hoarding is highly prevalent and heritable,with genetic factors accounting for approximately 50% of itsvariance [Iervolino et al., 2009]. Very few studies have examinedthe genetic architecture of hoarding, and their results have beenlargelyinconsistent[Zhangetal.,2002;Samuelsetal.,2007;Pertusaet al., 2010].We conducted a genome-wide association study for hoardingtraitsinalargecohortofCaucasiantwins(see[Iervolinoetal.,2009]for more details). A sub-sample of 3,410 participants had beengenotyped and was included in this study. Of these participants,2,350 were singletons (either MZ as MZ twins are geneticallyidentical, only one member of the twin pair was genotyped- orDZ without the co-twin), predominantly female (91.8%), with ameanageof56.8years(SD¼12.6;range¼17–85).AllparticipantscompletedtheHoardingRatingScale-Self-Report(HRS-SR[Tolinetal.,2010]),abriefself-administeredinstrumentconsistingoffiveitems (clutter, difficulty discarding, excessive acquisition, distress,andimpairment).Eachitemisscoredfrom0(none)to8(extreme)with a total score ranging from 0 to 40. Scores above 14 indicateseverehoardingwithsensitivity andspecificityof0.97.HRSscoreswereobtainedforeachmemberofaMZtwinpairbutonlyonetwinwasgenotyped. Inordertominimizemeasurementerrorandgetabetter estimate, the HRS-SR scores of each MZ twin pair weretreated as biological duplicates and averaged. The mean HRS-SRscore in our cohort was of 2.83 (SD¼4.2; range¼0–35).SubjectsweregenotypedusingeitherIllumina317K(n¼1,348)or 610K (n¼2,062) BeadChips. All subsequent analyses werecarriedoutusingPLINKsoftware[Purcelletal.,2007].Afterqualitycontrol procedure and imputation of genotypes (SupplementaryInformation), a dataset composed of 3,304 individuals and1,517,033 SNPs was available for analyses.A HRS-SR score adjusted by age and the first two principalcomponents of the population stratification analysis was used forassociation. To account for the dependence between related indi-viduals, we used the –qfam-total option with permutations de-signed to extract all association information from a family-basedsample [Purcell et al., 2007]. The –qfam procedure adopts thebetween/within model and performs a simple linear regression ofphenotype on genotype using a special permutation procedure tocorrect for family structure allowing to integrate both singletonsand related individuals in a same analysis. Finally, the inflationcoefficient factor after permutation was of 0.98 suggesting noinflation of the tests statistics.Although no SNP demonstrated evidence for associationat a genome-wide level of significance, two genomic loci on