OPN1SW and acute lymphoblastic leukemia: 103). Haematological malignancies such as relapsed CLL, follicular lymphoma and small lymphocytic lymphoma have been observed to depend on pre-B cell receptor signalling, which can also be seen in the majority of TCF3-PBX1 BCP-ALLs. The specificity of idelalisib to p110δ, results in a low toxicity profile, making it a promising therapeutic for TCF3-PBX1 BCP-ALL patients (Refs 104, 105).