MS-1020 is a cell-permeable inhibitor of janus kinase 3 (JAK3), strongly inhibiting constitutive autophosphorylation of JAK3 in L540 cells when used at 30-50 μM. It is without effect on other JAK isoforms and several other kinases, including Src, Lyn, Akt, EGFR, and ERK1/2. Through its effects on JAK3, MS-1020 blocks phosphorylation of downstream signal transduction and activators of transcription isoforms, reducing the expression of anti-apoptotic genes, leading to cell death.