We first asked if the X-linked modifier resided on the Xp or Xq arms and compared ZPBP2 methylation in fibroblast cell lines from four Turner syndrome patients with monosomy X (45, X), four individuals with monosomy for most of the p-arm and trisomy for the q-arm of X chromosome (referred to as 46,X,i(Xq)) and three females with trisomy X (47 or 48,XXX) (Fig. 2a). The gene discussed is ZPBP2; the disease is Turner syndrome.