Among the five parents carrying monoallelic germline variants, only one subject (mother of P3.1) had persisting JAK2-negative thrombocytosis since the age of 15 years and Crohn’s disease with arthritis and vitiligo (value of >1.000 × 109/L in more than one determination, normal the rest of complete blood count). The gene discussed is JAK2; the disease is Arthritis.