One very in favour of medium protein variant (7)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I (one), SO for very, in favour of: FOR and M[edium]. Not in Chambers, but Collins has “any of several forms of the same protein, derived either from the same gene or from different genes, each of which is similar in function to the others”. I reasoned it worked the same way as isotope, and couldn’t see any indication that it was not a thing.