With more genotype-phenotype correlation studies being performed, a positive genetic diagnosis will help the clinician to be aware of possible anomalies specific to that certain gene mutation. For example, an increase in hypertrophic cardiomyopathy is seen in people with a mutation of KRAS and an increased risk of juvenile myelomonocytic leukemia exists for a mutation of PTPN11. In the future, studies may lead to a targeted management of NS symptoms that depends on what genetic mutation a person has. Before birth
Prenatal features that might lead physicians to consider a diagnosis of Noonan syndrome include cystic hygroma, increased nuchal translucency, pleural effusion, and edema. Differential diagnosis
While Turner syndrome has similarities with renal anomalies and developmental delay, Turner syndrome is only found in females and often expresses differently. In Turner syndrome, there is a lower incidence of developmental delays, left-sided heart defects are constant and the occurrence of renal abnormalities is much lower.Other RASopathies

Watson syndrome - Watson Syndrome has a number of similar characteristics with Noonans Syndrome such as short stature, pulmonary valve stenosis, variable intellectual development, and skin pigment changes. Cardiofaciocutaneous (CFC) syndrome - CFC syndrome is very similar to Noonans Syndrome due to similar cardiac and lymphatic features. However, In CFC syndrome intellectual disability and gastrointestinal problems are often more severe and pronounced. Costello syndrome - Like CFC syndrome, Costello syndrome has overlapping features with Noonans Syndrome. However, the conditions can be distinguished by their genetic cause.
Mutations in the Ras/mitogen activated protein kinase signaling pathways are known to be responsible for about 70% of NS cases.Persons with NS have up to a 50% chance of transmitting it to their offspring. The fact that an affected parent is not always identified for children with NS suggests several possibilities:

Manifestations could be so subtle as to go unrecognized (variable expressivity)
NS is heterogeneous, comprising more than one similar condition of differing causes, and some of these may not be inherited. A high proportion of cases may represent new, sporadic mutations.Heterozygous mutations in NRAS, HRAS, BRAF, SHOC2, MAP2K1, MAP2K2, and CBL have also been associated with a smaller percentage of NS and related phenotypes.A condition known as "neurofibromatosis–Noonan syndrome" is associated with neurofibromin. Diagnosis
NS can be confirmed genetically by the presence of any of the known mutations listed above. However, despite identification of 14 causative genes, the absence of a known mutation will not exclude the diagnosis, as  more, as-yet-undiscovered genes can cause NS. Thus, the diagnosis of NS is still based on clinical features. In other words, it is made when a physician feels that a person has enough of the features to warrant the label. The principal values of making a genetic diagnosis are that it guides additional medical and developmental evaluations, it excludes other possible explanations for the features, and it allows more accurate recurrence risk estimates.