PCNA and chronic obstructive pulmonary disease: Data from the quantitative analysis further exhibited that the proliferation of ASM cells in small airway was increased with the progression of COPD diseases when it was ascertained by the percentage of PCNA positive ASM cells (Figure 3(e)), and the numbers of ASM cells per square millimeter in the area of ASM in small airway (Figure 3(f)), suggesting that the proliferative capacity of ASM cells was enhanced with the severity of airflow limitation of COPD.