...two patients with ESR1 mutations (in the absence of concurrent driver alterations in RB1, FGFR, CCNE2, and ERBB2) demonstrated progression on palbociclib and sensitivity to abemaciclib...In one patient, cfDNA obtained prior to palbociclib and fulvestrant exposure failed to reveal any ESR1 alteration. Following progression on palbociclib, and prior to sequential exposure to abemaciclib, an ESR1 Y537N alteration was identified....These results suggest that patients with HR+ MBC, ESR1 mutation, and clinical resistance to anti-estrogen treatment and palbociclib may be candidates for abemaciclib treatment.