PLK1 and neuroblastoma: To that end, we first immunoprecipitated (IP'ed) endogenous Elk-1 from SH-SY5Y neuroblastoma cells and showed that it interacts with all four mitotic kinases—Aur-A, Aur-B, Plk1, and Cdk1—albeit to a lower extent in the case of Cdk1; however, no interaction was observed when a control IgG was used in immunoprecipitation (IP lane vs. IgG lane; Figure 1(b)).