Finally, 9 potential NSCLC metastasis-associated proteins, including SMC5, PSIP1, KHSRP, VRK1, GTF21, PTRF, SNW1, VASP, DDX21, were selected to further validate the mass spectrometry results (Additional file 1: Figure S2A). The gene discussed is DDX21; the disease is non-small cell lung carcinoma.