The present invention relates to methods and pharmaceutical compositions for inhibiting lymphocyte proliferationin a subject in need thereof. In particular, the invention relates to a CTP synthase 1 (CTPS1) inhibitor for use in a method for inhibiting lymphocyte proliferationin a subject in need thereof. The invention also relates to a method for screening a plurality of test substances useful for inhibiting lymphocyte proliferationin a subject in need thereof comprising the steps consisting of i) testing each of the test substances for its ability to inhibit CTPS1 activity or expression and ii) identifying the test substance which inhibits CTPS1 activity or expression thereby to identify a test substance useful for inhibiting lymphocyte proliferationin a subject in need thereof.
