MYO1G is a member of class I unconventional myosins. Eighteen different classes of myosins are known. This behavior has not been observed in other myosins. The unconventional myosins also have divergent tail domains, suggesting unique functions. NM-IIB, and non-muscle myosins in general, are widely expressed in every tissue in humans. The now diverse array of myosins likely evolved from an ancestral precursor (see picture). In biological systems, molecular motors, like myosins in muscles, use chemical energy to create motion.