CTSD and colorectal carcinoma: In light of the above data, we can hypothesize that expression of Cathepsin-D in CRC cells follows overall S1P content and the S1P/Cer ratio, with the lowest mRNA expression in this enzyme concomitant with a low S1P/Cer ratio and S1P content in SPHK1-silenced cells and the highest protein expression under a high S1P/Cer ratio and S1P accumulation in SGLP1-silenced cells.