In distinction to the known RASopathies, which are single gene disorders, the 6p25.1p24.3 microdeletion syndrome arises via transcriptional dysregulation of multiple RREB1 target genes including FGFR4, HRAS and MAP2K2 that result in sensitization of the MAPK pathway to activation. The gene discussed is FGFR4; the disease is RASopathy.