Question about negative value in gep_scores matrix

[Tang-RH]

Dear authors: Thank you for your work on developing the nice tool cNMF. When I run the PBMC tutorial https://github.com/dylkot/cNMF/blob/master/Tutorials/analyze_pbmc_example_data.ipynb, I found that the gep_score ,the result matrix to select top genes in a GEP, had some negative values. And I saw the similar result in my own data. I want to confirm that if the gep_score matrix is one of the nmf result matrixes. If so, why there are some negative values in it. If not, what is it, and why can we use it to select the top genes in a GEP?