ICOS and Huntington disease: In agreement with previously published T-cell phenotypes from unaffected parents of ICOS-deficient patients (9) and consistent normal circulating CXCR5+PD-1+ T follicular helper (cTfh) cell frequencies (Table S1 in Supplementary Material), we found ICOS expression on subject 1/subject 2 cTfh cells, Tregs and naïve T cells to be indistinguishable from HD/subject 3 T-cell subsets (Figure 1B; Figure S2 in Supplementary Material).