A significant combined effect of maternal smoking and deletions in GSTM1 or GSTT1 in the infant or mother was observed on cleft lip/palate (CL/P) or oral cleft development in case-control studies in the US and the Netherlands14, 28, 29; however, no gene-smoking interaction effects have been identified in other studies30, 31. The gene discussed is GSTM1; the disease is cleft lip.