MUC1 and rheumatoid arthritis: Lee et al. studied 165 patients with CTD-ILD (41 RA, 53 systemic sclerosis, 56 inflammatory myopathy, 15 systemic lupus erythematosus or SJS), and also showed that the semiquantitative grades of ILD on the HRCT (grade 1, 0–25%; grade 2, 26–50%; grade 3, 51–75%; grade 4, 76–100%) were significantly proportional to serum KL-6 levels, from which grades could be successfully differentiated (grades 1 vs. 2, P = 0.022; grades 2 vs. 3, P < 0.001; grades 3 vs. 4, P = 0.002)17.