Regulators, scientists, clinicians and patient advocacy groups often cite ~7,000 as the number of rare diseases, or between 5,000 and 8,000 depending on the source. Why do estimates of the number of rare diseases vary so widely? One reason is the aforementioned lack of consistency in defining discrete disease entities and their incidence in different countries or demographics. Another reason is computational differences and imperfections in the current terminologies. Some terminologies do not include chromosomal disorders, or other structural variations such as inversions, while others do not include rare diseases with environmental causes, such as toxin exposure. For more information click here.