Huntington’sdiseaseiscausedbyanexpandedCAGtrinucleotiderepeatcodingforapolyglutaminestretchwithinthehuntingtinprotein.Huntingtinco-purifieswithasinglenovel40kDaproteindesignatedHAP40.RecombinantHAP40iscytoplasmicinthepresenceofhuntingtinbutisactivelytargetedtothenucleusintheabsenceofhuntingtin.TheseobservationssuggestthatHAP40contributestothefunctionofnormalhuntingtinandisacandidateforinvolvementintheaberrantnuclearlocalizationofmutanthuntingtinfoundindegeneratingneuronsinHuntington’sdisease.