I'm not sure what you want exactly. Are you after differential gene expression? In the case of GEO, you can simply use GEO2R to identify DEGs in most datasets. Let me know if you need anything else. Thanks
Even though I am not sure if I understand what you mean, here is my answer. As I understand it, you have a gene of interest and its expression in breast cancer samples and normal tissue samples. A fold change can be calculated by dividing the expressions, so the gene expression difference could be found that way. In this case, it represents breast cancer expression divided by normal tissue expression. You can then use log to obtain fold change as well. So that's how differential expression is calculated. Does that answer your question?
Jagajjit Sahu ,thanks for your answer, actually I want to know how to analysis the different expression of my interest gene between normal tissue and breast cancers by using some tools.
Sx Wang In that case, may I suggest for TCGA, use this webtool GEPIA 2.0, wherein you can compare the expression of gene(s) of interest between Tumor and Normal, and across different Tumor stages. (http://gepia2.cancer-pku.cn/#index ) However, if you're up to the task, I'm sure datasets in GEO have this function called GEO2R, wherein they give you basic code to load the dataset, extract gene expression data, and run DGE analysis. If you have a sample dataset, I can guide you further.