RASopathies comprise a group of clinically overlapping developmental disorders caused by genetic variations affecting components or modulators of the RAS-MAPK signaling cascade, which lead to dysregulation of signal flow through this pathway. Noonan syndrome and the less frequent, clinically related disorders, Costello syndrome, cardiofaciocutaneous syndrome, Noonan syndrome with multiple lentigines, and Noonan syndrome-like disorder with loose anagen hair are part of the RASopathy spectrum and share a recognizable pattern of multisystem involvement. This review describes the "Noonan syndrome-like" phenotype as a common phenotypic signature of generalized developmental RAS pathway dysregulation. Distinctive features of the different entities are revisited against the background of the understanding of underlying genetic alterations and genotype correlations, which has evolved rapidly during the past 20 years, thereby leading to suggestions regarding the nosology of RASopathies. K E Y W O R D S cardiofaciocutaneous syndrome, Costello syndrome, LEOPARD syndrome, Noonan syndrome, Noonan syndrome with multiple lentigines, Noonan syndrome-like disorder with loose anagen hair 1 | INTRODUCTION The term "RASopathies" was coined in 2009 (Tidyman & Rauen, 2009). It denotes a group of clinically overlapping multisystem disorders that share a common molecular pathogenesis in that the underlying genetic variations affect components or modulators of the RAS-MAPK signaling cascade and lead to dysregulation of signal flow through this pathway. No generally accepted definition and definitive delineation of RASopathies exist, so far. The major classes of disorders that have been grouped under this umbrella term are summarized in Table 1. This clinical overview is focused on Noonan syndrome (NS; MIM #PS163950) and the less frequent, clinically related disorders, Costello syndrome (CS; MIM #218040), cardiofaciocutaneous syndrome (CFCS: MIM #PS115150), Noonan syndrome with multiple lentigines (NSML; MIM #151100), Noonan syndrome-like disorder with loose anagen hair (NSLH; MIM #PS607721), collectively referred to in the following as "Noonan syndrome-like RASopathies (NS-like RASopathies)." They represent disorders that are caused by a mild to moderate and generalized constitutional RAS pathway dysregulation (Figure 1a) and have a multisystem phenotype. Based on the current pathogenetic concepts and understanding it can be concluded that the "NS-like" clinical picture represents the common phenotypic signature of a systemic RAS pathway dysregulation that starts in the embryonic period. This notion is supported by the fact that all genes known to cause NS-like RASopathies encode for components or modulators of the RAS-MAPK signaling cascade. However, this does not mean that disturbed signaling through MAPKs as the sole effectors can explain all the organ-specific pathophysiology. In fact, cellular pathways are interconnected and abnormal signaling in one of them