The organization of microtubules into a mitotic spindle is critical for animal cell proliferation and involves the cooperation of hundreds of proteins whose molecular roles and regulation are not fully understood. The protein product of the Drosophila gene abnormal spindle, Asp, is a microtubule-associated protein required for correct mitotic spindle formation. To better understand the contribution of Asp to microtubule organization during spindle formation, we reverse-engineered flies to express a version of Asp (Asp