This is an open-label, multicenter, international, randomized, Phase II clinical trial that
      will assess the efficacy and safety of letrozole in combination with afatinib(oral epidermal
      growth factor receptor (EGFR ) inhibitor) versus letrozole monotherapy for the first-line
      treatment of postmenopausal women with ER+, Human Epidermal Growth Factor Receptor 2 (HER2)
      negative advanced breast cancer with low ER expression.

      In order to assess the level of estrogen receptor (ER) expression we will use a
      semi-quantitative scoring system (McClelland, 1990) defined as :

      H-score = (% of cells stained at intensity category 1x1) + (% of cells stained at intensity
      category 2x2) + (% of cells stained at intensity category 3x3).

      This formula results in an H-score in the range of 0-300 where 300 equals 100% of tumor cells
      stained strongly (i.e., 3+). Low ER expression will be defined as tumor sample with H-score
      below 160 (Finn, 2009).

      All subjects who consented for the study must submit a tumor sample to the designated central
      laboratory for central confirmation of ER / Progesterone receptor (PR) and HER2 statuses and
      determination of the H-score. This will be assessed prior to randomization.

      Subjects with HER2 negative, ER+ advanced breast cancer with low ER expression defined as
      H-score between 1 and 159 will enter screening phase and perform the required screening
      assessments.

      Eligible subjects will be randomly assigned in a 1:1 ratio and stratified according to sites
      of disease (bone only disease vs. other) and prior administration of hormonal therapy in
      neo/adjuvant setting (Yes vs. No) to either:

      Arm A : Continuous regimen of oral letrozole 2.5 mg until progression of disease or any other
      study treatment discontinuation criteria.

      or Arm B : Continuous regimen of oral letrozole 2.5 mg daily plus oral afatinib 30 mg daily
      until progression of disease or any other study treatment discontinuation criteria.

      IN ADDITION the following applies whichever comes first:

        -  If the patients treated with the combination of afatinib and letrozole (arm B)
           discontinue the trial treatment (whatever the reason) before 30 November 2018, the
           patients from the other arm (arm A, letrozole alone) still on treatment will also be
           discontinued from the trial at the same time. They may continue receiving letrozole
           using commercial drug as standard of care according to their treating physician
           discretion.

        -  If the patients treated with afatinib and letrozole (arm B) have not discontinued the
           trial treatment by 30 November 2018, all patients currently on treatment in the trial
           (including the ones only treated by letrozole alone (arm A)) will be discontinued from
           the trial at that time. They may continue receiving their treatment if in alignment with
           their treating physician judgment as follows:

        -  Patients in arm A: may continue receiving letrozole using commercial drug as standard of
           care according to their treating physician discretion.

        -  Patients in arm B: may continue receiving afatinib in the context of alternative drug
           supply outside the clinical trial as appropriate according to local legislation.
           Additionally, they may continue receiving letrozole using commercial drug as standard of
           care according to their treating physician discretion.

      Once the patient is discontinued from trial treatment and has undergone the End of Treatment
      Visit, she will be permanently discontinued from the trial and treated as per local clinical
      practice.