Two cases with severe lethal course of Costello syndrome associated with HRAS p.G12C and p.G12D

► Two cases with severe neonatal manifestation of Costello syndrome. ► The two individuals carry the HRAS mutations p.G12C and p.G12D. ► Both had hypertrophic cardiomyopathy, tachyarrhythmia, edema, and respiratory distress. ► Our data provide evidence for a genotype-phenotype correlation in Costello syndrome.