TKT and leukemia: However, STK1 protein levels determined by the TK-ELISA in the different subgroups showed two different patterns i.e. in the lymphomas and leukemias most of the sera had STK1 protein levels above the cut-off (Fig 5C) (i.e. 30/38 and 4/5, respectively) similar to what was found with the STK1 activity.