Mutations in the Plk gene lead to instability of Plk protein in human tumour cell lines

It has been established that mutations in Drosophila Polo cause abnormaliti es in mitosis(1). In human cells, maximal Plk activity is reached in the M phase of the cell cycle, and the function of Plk is therefore considered to be required for mitotic cellular events such as spindle formation, chromos ome segregation and cytokinesis. Microinjection of antiPlk antibody into li ving cells has been found to induce a mitotic abnormality that contributes to the generation of aneuploidy(2), and this is an important finding in rel ation to tumour development. Indeed, previous studies have shown that the l evel of expression of a mitotic checkpoint gene, hsMAD2, is reduced(3) and that another checkpoint gene, BUB1, is mutated in certain human cancer cell s(4).