Chronic exposure to SWCNT induces malignant transformation and CSC formation in small airway epithelial cells. (A) Analysis of colony formation (left) and tumor spheres (right) in passage control SAC cells, chronic SWCNT- and asbestos-exposed SASW and SAAB cells after three weeks of culture. (B) Quantitative analysis of colony and tumor spheres. (C) Analysis of side population (SP) in SAC, SASW, and SAAB cells in the presence or absence of fumitremorgin c (FTC) using FACS. SP cells (box) are determined by their disappearance in the presence of FTC. (D) Quantitative analysis of SP subpopulation. Data are mean ± SD (n = 4). *p < 0.05 vs. control SAC cells.