Correction of spatial artifacts by local normalization. (A) Distribution of signal ratios along the Y- (left) and X-axis (right) of the microarray slide before normalization. The data are from the same array as in Figure 2 (left side). Only spots giving usable data are shown. The groups of spots separated by small gaps reflect the 12 × 4 sub-grids of the array, each printed with a different spotting pin. (B) Distribution of signal ratios as in (A) after local normalization of the data. (C) Overlay of the data from (A) and (B).