Additionally, an epigenetic study by Ouyang et al. (2013) showed that there were differences in methylation levels in the promoter regions for two genes involved in inflammatory response, interferon-γ (IFN-γ) and interleukin-4 (IL-4), in workers with isocyanate-asthma when they were compared to exposed workers without isocyanate-asthma or to non-smoking exposed workers. Here, IL4 is linked to asthma.