The main goal of precision medicine is to ascertain the correct diagnosis in an individual patient to deliver the most appropriate prognosis, counseling, and treatment. Accurate diagnosis is therefore the main prerequisite in precision medicine for clinical management of both Mendelian and non-Mendelian diseases. Precise diagnosis and medical management of human diseases, especially genetic ones, can be hampered by phenocopies, genetic and phenotypic heterogeneity, and coincidental co-occurrences.