Estrogen receptor mutations and uses thereof

A method of treating a subject having a breast cancer, the method comprising:

• acquiring knowledge of a presence of (a) a mutant estrogen receptor 1 (ESR1) polypeptide or a nucleic acid molecule encoding the mutant ESR1 polypeptide and (b) an Estrogen Receptor positive (ER+) breast cancer in said subject,
• wherein the mutant ESR1 polypeptide or the nucleic acid molecule encoding the mutant ESR1 polypeptide comprises a mutation chosen from one or more of:
• (i) one or more of mutations at amino acid position 311, 341, 350, 394, 414, 433, or 503, or 538 of SEQ ID NO:
• 2;
• (ii) a deletion of nucleotides 1046-1051 of SEQ ID NO:
• 1;
• (iii) an insertion of a histidine as shown at amino acid position 349 of SEQ ID NO:4;
• (iv) an insertion between amino acid positions 344 and 345 of SEQ ID NO:
• 2;
• or
• (v) a tyrosine to cysteine substitution at amino acid position 537 (a Y537C) of SEQ ID NO:
• 2;
• and
• responsive to said knowledge, administering to the subject an effective amount of an estrogen receptor modulator set forth in Table 5,
• thereby treating the breast cancer in the subject.