I like the richness index that is implemented in the "rarefy" function of the "vegan" package for R. For example, if one calculates the richness index on a scale (subsample size) of 10 for a given gene, it is basically asking the question of how many samples would seen be expessing that gene if the total number of counts for that gene were 10. The lower the richness, the higher the tissue specificity. To normalise the expression table before calculating the richness, I downsample all the libraries at the same level. Note as well that a meaningful richness index can not be calculated on a scale that is lower than the total number of counts for a given gene. Thus, in the example above, genes with less than 10 counts in total over all the sample should be excluded.