This clinical trial is studying two combinations of medicines for people with a certain type of breast cancer called ER+/HER2- with an ESR1 mutation. ER+ means the cancer grows with estrogen, HER2- means it doesn't have too much of a protein called HER2, and ESR1 mutation is a change in the gene related to estrogen. The study is for women and men who have already tried a treatment called ribociclib or palbociclib. Participants will receive either lasofoxifene and abemaciclib or fulvestrant and abemaciclib, which are given in different ways (oral or injection).

• The study includes oral and injection treatments, with some requiring a daily intake.
• Participants must have specific cancer characteristics and prior treatments to join.
• Important factors to consider are treatment history, current health conditions, and willingness to comply with study rules.