This is a data-driven, retrospective, longitudinal, population- based, observational, multi-centered study using secondary data captured from congruent electronic health records (EHRs).

Patients with pathologically documented an initial breast cancer diagnostic (early breast cancer (BC) or locally advanced BC or de novo metastatic BC) with documented human epidermal growth factor receptor 2 (HER2) and estrogen receptor (ER)/progesterone receptor (PgR) expression status at the time of diagnosis.

Data to determine the primary endpoint is estimated to be derived from the EHRs of over 2,000 patients that have an initial diagnosis of early BC, or locally advanced BC, or de novo metastatic BC (mBC) between the 1st of January 2005 and the 31st of December 2021, and who had at least one subsequent relapse until the 31st of December 2021 in at least 10 clinical centers.

The secondary endpoints utilize a larger collection of data from over 30,000 patients that have an initial diagnosis of early BC, or locally advanced BC, or de novo mBC between the 1st of January 2005 and the 31st of December 2021.

Inclusion Criteria:

1. Male or female patients who have at least 18 years of age at enrollment.

2. Patients with initial diagnosis of early BC or locally advanced BC or de novo mBC between the 1st of January 2005 and the 31st of December 2021

3. Pathologically documented BC for:

   • HER2 receptor expression with a validated assay according to American Society of Clinical Oncology - College of American Pathologists (ASCO/CAP) recommendations at the time of diagnosis. HER2 receptor expression can be: o HER2-positive expression: defined as immunohistochemistry (IHC) 3+ or concurrent IHC 2+ with in situ hybridization (ISH) positive or HER2-low expression: defined as IHC 2+ with ISH-negative or IHC 1+ with ISH-negative or untested) or HER2-negative expression: defined as IHC: 0.
   • Estrogen receptor [ER]- and/or progesterone receptor [PgR] with a validated assay according to ASCO/CAP guidelines at the time of diagnosis during early and/or advanced setting. ER/PgR expression can be: positive: ER or PgR ≥1% or negative: ER and PgR <1%

4. Electronic Health Records (EHRs), with guaranteed data to meet requisites, about clinicopathological characteristics, type of surgery, treatment management, disease outcomes, and genomic profile. Centers that agree to participate must commit to include all subjects who meet the inclusion criteria, in order to reduce possible selection bias.

Exclusion criteria:

1. Medical charts at Hospital that cannot guarantee reliable and congruent EHRs.
2. If sufficient data cannot be obtained from EHRs.

Patients with an initial confirmed diagnostic of breast cancer (early or locally advanced or de novo metastatic) managed in clinical practice during the predefined timeframe in the selected hospitals.