Summary: SScore is an R package that facilitates the comparison of gene expression between Affymetrix GeneChips using the S -score algorithm. The S -score algorithm uses probe level data directly to assess differences in gene expression, without requiring a preliminary separate step of probe set expression summary estimation. Therefore, the algorithm avoids introduction of error associated with the expression summary estimation process and has been demonstrated to improve the accuracy of identifying differentially expressed genes. The S -score produces accurate results even when few or no replicates are available. © 2006 Oxford University Press.