cBioPortal mRNA expression Z-scores information

1. [The paper](https://pmc.ncbi.nlm.nih.gov/articles/PMC5957518/) for the `brca_tcga_pan_can_atlas_2018` study that I used as example in #7:

2. Sections of cBioPortal FAQ about Z-scores: https://docs.cbioportal.org/user-guide/faq/#what-are-mrna-and-microrna-z-scores
> What are mRNA and microRNA Z-Scores?
> Is there any normal RNA-seq data in cBioPortal?
> How is TCGA RNASeqV2 processed? What units are used?

3. cBioPortal file formats
- Supported `stable_id` values for `MRNA_EXPRESSION` (suffixes of `molecularProfileId`): https://docs.cbioportal.org/file-formats/#supported-stable_id-values-for-mrna_expression
- z-score instructions: https://docs.cbioportal.org/file-formats/#z-score-instructions

4. Z Score normalization script
https://docs.cbioportal.org/z-score-normalization-script/#z-score-normalization-script