Despite identification of four causative genes, the diagnosis of Noonan syndrome is still based on clinical features. In other words, it is made when a physician feels that a patient has enough of the features to warrant the label indicating association.
The patient can be tested for mutations in the PTPN11, SOS1, or KRAS genes, however absence of a mutation will not exclude the diagnosis as there are more as yet undiscovered genes that cause NS.
The principal values of making such a 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.
Further Reading
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"Noonan syndrome"
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