HHIP and chronic obstructive pulmonary disease: In particular, they tested it by examining how genetic perturbations affected the relationships between the biomarkers: in their example, they compared groups of subjects homozygous for risk and non-risk alleles at known GWAS SNPs and saw that few of the discovered interactions were different between homozygotes for alternative alleles of COPD GWAS regions near HHIP and FAM13A (two SNPs associated with COPD).