This paper investigates the distinction between nominative and genitive subject complement clauses in Kazakh (Turkic) to determine whether these clauses have the same syntactic structure or whether they are derived independently using different strategies. Based on novel data, the paper shows that Kazakh morphologically distinguishes anaphoric and unique definiteness (in the sense of Schwarz 2009), and that genitive marking on the subject of complement clauses is determined by anaphoricity of the subject. Therefore, nominative and genitive subjects are in complementary distribution: anaphoric subject DPs are in the genitive, while pseudo-incorporated and unique definite subjects are nominative-marked. This distinction can be accounted for by analyzing the anaphoric subject undergoing a semantically motivated movement to the edge of the complement clause, where it is assigned lexically-governed genitive. The analysis contributes to the cross-linguistic study of morphological case assignment by showing that genitive can be a lexical case, and also by demonstrating that definiteness marking can drive differential case marking.