Penalty for unexpressed genes

Hi all,

I have expression data of two tissues and I have come up with TSI – Tissue specific Index which gives mostly the positive values. My task is looking into this score I should be able to say if the gene is highly expressed or lowly expressed in these two tissues.

Can anyone help me with solving this? I need to know is there any penalty based scoring index or method which can be used to give an solution to the problem by adding penalty to an unexpressed gene?