ALK and anaplastic large cell lymphoma: Another mechanistic study of PD-L1 expression in NPM-ALK fusion positive anaplastic large cell lymphoma found that PD-L1 expression was induced mainly through the MEK-ERK and STAT3 signaling pathways (Figure 4B), suggesting that the expression of PD-L1 in ALK fusion is regulated in different ways depending on tumor cell types or differences in ALK fusion partners (38, 39).