BCL2L1 and neoplasm: Opposing anti-apoptotic Bcl-XL (i.e., the long isoform), Bcl-XS (the short isoform) is a pro-apoptotic splice variant of Bcl-X, the Bcl-XL/Bcl-XS ratio being defined by the cell type and cell differentiation, which are dependent (e.g., non-transformed versus tumor cells) by numerous determinants including transcription factors and cytokine signaling [72].