HAP1 (huntingtin associated protein) has previously been found to interact with huntingtin (htt) in a glutamine length dependent manner and has been proposed to play a role in the cell specific neurodegeneration observed in Huntington's disease (HD), We have isolated mouse HAP1 (hap1) and have shown that expression is not enriched in areas specifically affected in HD, We hale used the yeast two hybrid system to demonstrate that htt amino acids 171-230 are necessary for the hap1-htt binding and that hap1 does not interact with the transgene exon 1 protein in a transgenic model of HD. (C) 1998 Federation of European Biochemical Societies.