MTOR and esophageal squamous cell carcinoma: Another study showed that LncRNA HLA complex P5 (HCP5) promoted malignant behaviors of ESCC through regulating the miR-139-5p/phosphodiesterase 4A (PDE4A) axis and activating the PI3K-AKT-mammalian target of rapamycin (mTOR) signaling pathway (Xu et al., 2021b).