ESR1 and neoplasm: Zhang et al. were the first to describe ESR1 mutations in 1997, and since then, multiple resistance-culprit LBD mutations have been recognized and studied, not only in tumor tissue but also in circulating tumor cells (CTCs) and circulating tumor DNA (CtDNA) using newer more sensitive techniques such as the digital PCR (dPCR) or droplet digital PCR (ddPCR) (80).