Yes, MIOSES is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, O 1, S 1, E 1, S 1.

Is it a valid word?

MIOSES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(genetics) cell division that produces reproductive cells in sexually reproducing organisms; the nucleus divides into four nuclei each containing half the chromosome number (leading to gametes in animals and spores in plants)
reflex contraction of the sphincter muscle of the iris in response to a bright light (or certain drugs) causing the pupil to become smaller

Also: reduction division, light reflex, pupillary reflex.