Such S1P-governed chemotaxis requires specific S1P receptor expression on immune cells and has led to the development of several S1P receptor modulators, one of which is FDA approved for clinical applications in multiple sclerosis therapy: Fingolimod (FTY720), an S1P analogue that promotes an agonistic activation and subsequent internalization of S1P1 on lymphocytes, leads to lymphopenia since deficient S1P1 surface expression blocks S1P-governed lymphocyte egress from secondary lymphoid tissue (reviewed in Reference [26]). This evidence concerns the gene MBTPS1 and lymphopenia.