P2RY8-CRLF2 involves fusion due to an interstitial deletion of the pseudo-autosomal region 1 (PAR1) centromeric of CRLF2 in chromosomes X and Y. The latter is the predominant abnormality in Down syndrome associated B-ALL (DS-ALL) being present in ~50% of such cases [11]. The gene discussed is CRLF2; the disease is acute lymphoblastic leukemia.