Although decrement slight decrease of cellular S1P after FTY720 treatment was observed, we would exclude the possibility that the accumulation of ceramide in Kasumi-1 cells resulted from the direct inhibition of SPHK1 by FTY720, asthe amount of increased ceramide far exceeded the amount of decreased S1P and we did not observe increase of sphingosine after FTY720 treatment, as reported in prostate cancer cells [11] and NK-cell leukemia [6]. Here, SPHK1 is linked to aggressive NK-cell leukemia.