ALK and prostate cancer: In fact, as mentioned above, the FDA has approved several ctDNA tests as companion diagnostics [45,46,47]: detection of BRCA1/2 alterations for the use of the poly (ADP-ribose) polymerase (PARP) inhibitor rucaparib in ovarian cancer; BRCA1/2 and ATM mutations for the use of the PARP inhibitor olaparib in prostate cancer; ALK and EGFR alterations to be treated with the ALK inhibitor alectinib or the EGFR inhibitors gefitinib, erlotinib, and osimertinib in NSCLC; and a variety of PIK3CA alterations to be treated with the PIK3CA inhibitor alpelisib in breast cancer.