This observation was validated by absolute quantification of Axl by qRT-PCR (S3B and S3C Fig), comparing Axl mRNA expression in the neuroblastoma cell lines to the microglial cell line CHME3 used by Meertens et al. The results indicated very robust expression of Axl in CHME3 cells (~16000 copies of Axl mRNA per 20 ng of total RNA). The gene discussed is AXL; the disease is neuroblastoma.