A group of substances is defined by REACH as substances that have "(1) a common functional group; (2) the common precursors and/or the likelihood of common breakdown products via physical and biological processes, which result in structurally similar chemicals; or (3) a constant pattern in the changing of the potency of the properties across the category." (REACH Regulation EC No 1907/2006, Annex XI, Section 1.5) . In other words, the term "group" and "category" are used interchangeably in this article. As an example, two widely regulated groups of substances that fulfil all three aspects of grouping substances in the REACH regulation are dioxins (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-furans) and polychlorinated biphenyls (PCBs) , which are composed of about 210 and 209 individual congeners, respectively .