PIK3CD and acute lymphoblastic leukemia: In fact, the absence of the term “pre-BCR” in GO/KEGG databases obscured the assignment of some common genes that actually belong to the pre-BCR pathway, such as VPREB1, IGLL1, ZAP70, CD79A, and CD79B. More importantly, the PI3K signaling pathway has been shown to serve as the central downstream effector of the pre-BCR signaling pathway, and restoration of PI3K signaling pathway activation can rescue the loss of tonic pre-BCR signaling.20,21 Taken together, PI3K pathway involvement should be considered one of the characteristics for the pre-B stage of MEF2D fusions of BCP-ALL.