SPHK1 and breast cancer: In addition, MCF-10A cells that have high SPHK2 levels, responded to SKI-5C and DMS treatment equally, suggesting that inhibition of SPHK2 had little or no effect on cell viability (Supplementary Data Fig. 4A) These results indicate that the inhibition of SPHK1 activity by SKI-5C effectively impairs the growth potential of breast cancer cells.