Microtubules (MTs) are highly dynamic polymers that provide tracks for cargo transport within the cell, establish cell polarity, aid in cell motility, and form the mitotic spindle. MTs are nucleated from centrosomes and their plus ends exhibit dynamic instability. These dynamics are tightly regulated by Microtubule Associated Proteins (MAPs). One MAP family of particular interest is the highly conserved XMAP-215 family of polymerases. Members of the XMAP-215 family contain a varying number of N-terminal hexa-HEAT repeats known as TOG (tumor over-expressed gene) domains that bind to tubulin heterodimers. Minispindles (Msps), the XMAP-215 Drosophila homologue, localizes to MT plus ends to enhance MT polymerization and regulate the mitotic spindle morphology. While much of Msps’s structure has been solved, the vital C-terminal region structure and function remains a mystery. Furthermore, the C-terminal region of Msps may contain an additional 6th TOG domain. To determine the cellular role of this domain we utilized live cell fluorescent microscopy, immunohistochemistry, and mutagenesis assays to test the hypothesis that the hypothesized 6th TOG domain of Msps is necessary for functional MT polymerization and mitotic spindle assembly. Depletion of endogenous Msps and subsequent expression of a truncated Msps construct lacking the proposed 6th TOG Domain results in aberrant localization of the Msps protein, significantly slower MT growth velocities, and abnormal spindle phenotypes. This proposed 6th TOG domain contains highly conserved residues, which we hypothesize are required for Msps localization and function. To test this hypothesis we will generate a suite of constructs that have key conserved residues mutated and test which lesions phenocopy deletion of the entire domain.