The words «не кто иной, как» и «не что иное, как»: grammatical specialty

The article deals with the differentiation of the invariant parts of speech that do not have affixes fixing their semantic differences formally. The author focuses on the word «не кто иной, как» и «не что иное, как», the grammatical specialty of which, in spite of lexicographic information, raise reasonable doubts. The research is conducted on the basis of the modern Russian language.