The association between MYCN amplification (an important prognostic factor in NB) and PHOX2B expression in human NB cell lines [23], the down-regulation of MYCN expression after an RA challenge [48], [49], the up-regulation of PHOX2B associated with MYCN over-expression, and the down-regulation of PHOX2B and Mash1 when siRNA is used to inhibit MYCN [23], all support the hypothesis that a high PHOX2B expression level is due to the direct regulation of PHOX2B by MYCN, and we are currently investigating whether the down-regulation of PHOX2B is related to a reduction in MYCN expression. Here, MYCN is linked to neuroblastoma.