Possible causes of unexpected serum KL-6 elevation in patients with ILD include acute exacerbation of ILD, exacerbation of HP by increased antigen exposure, the development of PCP, CMV pneumonia, drug-induced pneumonitis, or adenocarcinoma of lung, breast, pancreas, ovary, colon and liver. Here, MUC1 is linked to hypersensitivity pneumonitis.