STK11 and non-small cell lung carcinoma: However, heterogeneity of immune checkpoint protein expression is a feature of NSCLC that limits the broad effectiveness of these therapies (Gettinger et al., 2015), with the absence of PD-L1 expression identified in both human and murine tumors with oncogenic KRAS and loss-of-function LKB1 mutations (Calles et al., 2015; Koyama et al., 2016; Skoulidis et al., 2015).