PAK1 and Huntington disease: Additionally, the PAK1 Interacting Protein 1 (PAK1IP1), which inhibits the activation of PAK1 by CDC42 through its interaction with the N-terminus of PAK1 (Xia et al, 2001), was upregulated in the brain of HD patients in all studies (Z-ratios: 1.88; 1.10; 1.98).