In order to identify whether Id specifically regulate genes controlling breast cancer metastasis, GSEA analysis was performed with a collection of custom “metastasis gene sets.” This collection (Table 1) consists of several metastatic signatures from the C2 collection (MSigDB database; Supplementary Table 4), combined with a list of custom gene sets described in major studies (Dontu et al., 2003; Kang et al., 2003b; Minn et al., 2005a,b; Tang et al., 2007; Padua et al., 2008; Bos et al., 2009; Charafe-Jauffret et al., 2009; Liu et al., 2010; Aceto et al., 2014) as shown in Figure 3C. Here, ID1 is linked to breast cancer.