Down syndrome vs Down's syndrome

Down syndrome and Down's syndrome both are nouns.

Word	Noun	Adjective	Verb	Adverb
Down syndrome	Yes	No	No	No
Down's syndrome	Yes	No	No	No

As nouns, Down syndrome and Down's syndrome are synonyms defined as:

Down syndrome and Down's syndrome: a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation

Other synonyms of Down syndrome include mongolianism, mongolism, trisomy 21.

Down syndrome (noun)	Down's syndrome (noun)
a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation	a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation