What does "ALT+ cancer cells", the plus sign mean? In many scientific papers, there is a plus sign:


  ... T-circles have also been detected in ALT+ (telomerase-negative) cells9 and ...


What does the "+" mean? Does it mean differentially expressed?
Here are a couple of papers (A and B) that explain ALT.  


  Telomere maintenance is a hallmark of cancer as it provides cancer cells with cellular immortality. A significant fraction of tumors uses the alternative lengthening of telomeres (ALT) pathway to elongate their telomeres and to gain an unlimited proliferation potential. Since the ALT pathway is unique to cancer cells, it represents a potentially valuable, currently unexploited target for anti-cancer therapies.
  B


The ALT+ denotes cell lines that are positive for the ALT pathway.