To identify the functional consequences of reduced CD28 expression, we compared subject 1/subject 2 T-cell proliferative responses against HD/subject 3 cells and also against cells from three additional subjects (subjects 4 through 6) with heterozygous ALPS5-associated CTLA4 mutations (CTLA4wt/mut, Table S1 in Supplementary Material). Here, CD28 is linked to Huntington disease.