Biology mistake in case of main protein (9)

'protein' is the definition.
(myoglobin is a kind of protein)

'biology mistake in case of main' is the wordplay.
'mistake' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'case of' means to remove the middle letters (outsides of).
'main' with its middle removed is 'mn'.
'biology' is an anagram of 'yoglobi'.
'yoglobi' put inside 'mn' is 'MYOGLOBIN'.