Expression of housekeeping genes across normal tissues. (A) Expression levels of the top 20 housekeeping genes, as ranked by average expression intensity, are shown. Three commonly used housekeeping genes, GAPDH, ACTB and UBC, are among this list. (B) Expression patterns for commonly used or commercially available housekeeping genes that do not meet the criteria defined here for housekeeping genes are shown. The length of the horizontal bars represents the expression intensity of the genes in microarray.