PPM1K and chronic obstructive pulmonary disease: In particular, we found: 1) The genes in the 4q COPD risk region are part of a dense co-expression network, leading to the discovery of multiple co-expression links between COPD GWAS candidate genes (CCG) (Section 2.2); 2) a network module of co-expressed genes including HHIP, BTC, NPNT and PPM1K (Section 2.3); 3) the existence of a differential co-regulatory sub-network between individuals with COPD and healthy controls, including FAM13A, PPA2, PPM1K and TET2.