PIK3CA and breast cancer: At present, many gene loci in ctDNA are diagnostic markers for breast cancer, such as PIK3CA, BRCA2, NBNPTEN, ESR1, AKT1, HER2, TP53, and GATA3 genes (Walsh et al., 2006; Berns et al., 2007; Jones et al., 2010; Janku et al., 2012; Li et al., 2013; Hosoda et al., 2014; Jeselsohn et al., 2014; Kuchenbaecker et al., 2017).