PIK3CA and breast cancer: As depicted in Fig. 4, a total number of 4 genes, namely ATM, FGFR3, TP53, and PIK3CA, are repeatedly mutated in all CTCs from our study and, considering that in all instances we evaluated BC patients with tumor progression and metastatic disease, we postulate that the expression of those major gene variants in CTCs could reflect their propensity to the metastatic activity.