To date, the experiments in yeast have been successful primarily in distinguishing what is not important. Divergent tubulin sequences are functionally interchangeable, interesting domains of tubulins can undergo dramatic alteration without affecting substantially the function of the protein, and the cells can assemble sufficient microtubule organelles from half the normal amount of tubulin. These results are with a few genes, in one organism, and the extent to which they can be extrapolated to other organisms with more complex complements of tubulin genes--in particular, metazoa--is not known. It is extremely unlikely that all of the suggestions that tubulin sequences and tubulin quantities participate in regulating microtubule function are erroneous. But the yeast system does offer us the opportunity to look past these obvious regulators, and to detect more subtle interactive elements that may be crucial for normal function. That is the immediate goal of our next experiments.