In order to examine the intracellular distribution of pEGFR in the gefitinib-sensitive NSCLC cell line PC9 or the gefitinib-resistant cell line QG56, in the absence of EGF stimulation, each cell line was double-labeled either with antibodies specific to pEGFR or with those specific to cathepsin D and lysosomal integral membrane protein(LIMPII) (Fig. 1). The gene discussed is CTSD; the disease is non-small cell lung carcinoma.