TCL1 (T cell leukaemia/lymphoma 1) protein is a 14 kDa proto-oncogene that regulates intracellular signalling in T cells. Its gene, located at 14q32.1, is overexpressed in T prolymphocytic leukaemia (T-PLL) due to chromosomal rearrangements that place it next to regulatory elements of the T-cell receptor. TCL1 activates the Akt pathway, promoting phosphorylation of key substrates in cell differentiation, growth and survival. Although involved in T tumours, it is also expressed in normal immune cells, such as thymic precursors and immature B cells. Its overexpression is linked to premalignant clonal expansion and mature T leukaemias.