To access the data, click or select the words “Appendix 2.” This will initiate the download of a compressed (pdf) archive that contains the file. Multiple probe sets corresponding to a single gene were combined by selecting whatever probe set gave the highest value in a particular cell, and their median expression was calculated. From the distribution of median expression levels over all cells on the x-axis (logarithmic) an initial peak of low expressed genes (probably off) on the left can be observed followed by a tail on the right side (probably on). Red lines represent chosen threshold levels.