There are some tips for finding good primary and secondary sources. The primary source is the object of the topic or the original source of any topic provided for you. These include historical documents, legal papers, and results of scientific researches or experiments, statistical data. The secondary source is something derived from the primary source. This means that you are quoting the primary source. Such quoting includes comments, suggestions and recommendations on the primary source. 

In your subject matter, you want to make the article “Biopsies Found to Provide Only a Snapshot of Tumor Diversity” written by Ed Yong as your secondary source. The article focuses on how ‘hard’ treating cancer is. This is a good secondary article if it quotes a primary article or stated something out of a primary article. You can look for an article that talks about something related on cancer that was in the paper. Looking at Yong’s article, some schools are noted there. You can look for the article in their website though.

Since they are looking for possible biomarkers, you can look for an article concerning biomarkers.

Primary source topics: Biomarkers on Cancer cells

Secondary Source topics: Biomarker diversity; Strength of biomarkers; Compatibility of Biomarkers; Costs; Availability of Biomarkers.