The aetiology of the OES/ECCL spectrum is related to the low percentage of mosaicism of activating pathogenic variants identified in FGFR1 (8p11.23, #136350) and KRAS (12p12.1, *190070) genes in DNA derived only from affected tissues [7]. This evidence concerns the gene FGFR1 and Toriello-Lacassie-Droste syndrome.