**1. Introduction**

Lung cancer is the leading cause of cancer-related death in the world. Despite significant advances in the diagnosis and treatment of the disease over the past 20 years, its prognosis is still very poor, with the overall 5-year survival rate staying at 15%–20% [1]. The prognosis of cancer patients is mostly determined by disease stage. The occult metastatic spread of cancer cells to surrounding tissues in more than 50% of lung cancer patients at the time of diagnosis affects a poor prognosis. The majority of patients undergoing curative surgical resection at an early stage and, if necessary, adjuvant chemotherapy have achieved favorable long-term survival. Patients with surgically resected stage IA, stage IB, and stage II non-small cell lung cancers (NSCLCs) had an overall five-year survival rate of 83%, 69%, and 48%, respectively [2]. Targeted therapy has a great effect on the prognosis

of specific patients; however, it is applicable to only about 10%–20% of patients. Accordingly, it is important to identify novel diagnostic and therapeutic biomarkers for the detection of early-stage lung cancer and for the development of new molecular-targeted therapies for NSCLC.

Runt-related transcription factor 1 (RUNX1) is one of the RUNX family proteins (RUNX1, RUNX2, and RUNX3), which forms a heterodimeric complex with the core binding factor β (CBFβ), resulting in enhanced transcription of the RUNX gene family by stimulating the DNA binding ability and stability of the family proteins [3,4]. RUNX1 is essential for hematopoiesis and is involved in the generation of hematopoietic stem cells. Mutations and translocations in *RUNX1* are well established as causes of myelodysplastic syndrome or acute myelogenous leukemia [5–7]. Gain- or loss-of-function mutations of *RUNX1* have also been reported in various solid tumors. Missense mutations of *RUNX1* were reported in luminal-type breast cancer [8], and loss-of-function somatic mutations or deletion of *RUNX1* have been reported in breast cancer and lung cancer [9,10].

To understand the clinicopathological significance of *RUNX1* in NSCLC, we analyzed the methylation status of *RUNX1* in different types of samples from a total of 118 NSCLC patients and 60 healthy individuals. The prediction performance of classifiers was validated in The Cancer Genome Atlas (TCGA) lung cancer. Expression levels of RUNX1 were also analyzed using HT-12 array and immunohistochemistry in tissue specimens from 42 and 409 NSCLC patients, respectively.
