## 3 Results  
Physiological Low Levels of ABCB1 Expression in Lung Tissue Are Significantly Downregulated in Cancer Cells  
First, the level of expression of the \(ABCB1\) gene was compared in different organs (Figure 1), with a particular emphasis on lung tissue (Figure 2). For this purpose, the resources of the TNMplot and UALCAN databases were used. The presented data confirmed the significant variation in the expression of this gene, depending on the type of tissue (Figure 1A,B). Moreover, for almost all analyzed tissues, the neoplastic process was associated with a significant change in the amount of mRNA in the cells, with the exception of colon and gastric cells (Figure 1B). For most tissues, including lung tissues, the level of expression was higher in normal tissues than in neoplastic ones. This difference was highlighted in the adrenal glands and some kidney tumors. In some cases, an increase in the expression of the \(ABCB1\) gene was observed in the tumor tissue compared to its normal cells. Examples include liver cancer and papillary renal cell carcinoma. Noticeable differences between the data obtained from independent databases may have resulted from the diversity of the histological types and subtypes included in these sources. As shown in Figure 2A,B, the correlation between the expression of the \(ABCB1\) gene and the malignant transformation of lung cells was present regardless of the histological type of the tumor. This correlation was present in both the glandular type (\(p<0.001\)) and the squamous cell type (\(p<0.001\)). In addition, there was also a difference in the level of expression of this gene between cancer and adjacent normal lung tissue (\(p<0.001\), Figure 2C). These observations are clinically significant due to the results of the evaluation of the correlation between the amount of mRNA of the discussed gene and the level of the protein encoded by this gene. As shown in Figure 3, there was a statistically significant correlation between the expression coefficient for mRNA and the amount of P-gp protein. Moreover, this dependence was proportional and occurred in both adenocarcinoma (R (Spearman) = 0.44, \(p<0.001\); R (Pearson) = 0.62, \(p<0.001\); Figure 3A) and squamous-cell lung cancer (R (Spearman) = 0.49, \(p<0.01\); R (Pearson) = 0.50, \(p<0.01\); Figure 3B). The presented data came from the cBioPortal database collected for this subgroup. Figure 1: \(A\)_BCRI_ expression in pan-cancer. (A) Differential \(A\)BCRI expression levels in tumors and normal tissues. Blue box plots—normal samples; red box plots—tumor samples; log2 (TPM + 1)—log2 of transcripts per million transformed to positive values (TPM + 1) (UALCAN). (B) Comparison of ARCBI expression between cancer and normal tissue. A red \({}^{*}\) marks those tissues for which the differences in the expression levels were statistically significant (TNMplot). Time Elapsed since Smoking Cessation/Quitting Affects P-gp Gene Expression in Ex-Smokers with Squamous-Cell Lung Cancer  
Cigarette smoking is invariably one of the main factors predisposing individuals to the development of lung cancer, and among smokers there is a tendency to develop the squamous histological type of this cancer. Nonsmokers are more likely to be diagnosed with the second type of NSCLC, adenocarcinoma [29]. Therefore, in a further part of the study, it was checked whether there is a relationship between smoking status and the amount of the \(ABCB1\) gene transcript in the cells of both histological types of lung cancer. It was observed that regardless of the status of smoking addiction, the amount of mRNA in adenocarcinoma cells remained at a low level (\(p<0.001\)). In contrast, the time that had elapsed since smoking cessation in ex-smokers seemed to affect \(ABCB1\) expression in squamous-cell carcinoma. A longer time since smoking cessation had a statistically significant effect on the increase in the transcript level of the studied gene (\(p=0.0228\), Figure 4B). Figure 2: Comparison of \(ABCB1\) gene expression between lung cancer and normal tissue.