Percent of Tumor vs Percent of cells

Does anyone know:

This might sound kind of dumb, but after reading a previous post I thought about this:

When we are described as ER % positive - does that mean a percentage of the tumor is positive or percentage of every cell is positive?

for example: If I were ER 5% positive; Is 95% of the tumor ER negative or are the cancer cells themselves 95% ER negative.

Thanks!
Maria