The relationship between mean and variance of protein expression. (A) An embryo image (bd5) divided into a grid, with 20 divisions along the anterior–posterior axis and 10 divisions along the dorsal–ventral axis. For the blue and red channels (representing Bicoid and Caudal protein, respectively), expression is relatively constant within each grid box. This is less true for the green channel (representing Even-skipped protein). (B) For each box we compute the mean and variance of the per-nucleus expression values for all nuclei within the box. If expression variance scales with mean, as observed by Bar-Even et al. (2006) for S.cerevisiae, then we should see a linear relationship between log variance and log mean, with a slope of approximately one. This is consistent with the data from this embryo. Best fit lines have slopes of 1.06 (blue channel), 0.94 (red channel) and 1.73 (green channel). The larger slope for Even-Skipped likely results from the spatially non-uniform expression pattern within each grid cell, adding another source of variance to the data besides real biological variability, rather than a true violation of the proportionality of mean expression and the variance of expression. Similar results were obtained from other embryos.