In previous reports, excluding patients with Down syndrome, the incidences of point mutations of JAK2 exons 16, 20, and 21 causing gain of function have been associated with IKZF1 deletion and P2RY8-CRLF2 fusion: 87.5% of patients with JAK2 mutations had IKZF1 alterations (P = 0.001) [18] and 30–100% of patients with JAK2 mutations had P2RY8-CRLF2 fusion [21–25]. This evidence concerns the gene CRLF2 and Down syndrome.