The advent of high-throughput array technology has now made it possible to collect data on thousands to tens of thousands of genes simultaneously. However, methods for detecting the genuine changes in the gene expression levels in cells or tissues are still evolving. [1][2][3][4][5][6][7][8][9][10][11] A straightforward method for comparing two expression levels of genes is the traditional two-sample t-test. The basic problem with the t-test in microarray experiments, however, is that the repetition is restricted within a small number in most cases, because experiments are costly or tedious to repeat. Although the importance in replication has been illustrated, 1,3,6,9 situations often arise where only single or duplicate experiments for each condition are allowed.The purpose of this paper is to put forward a method for testing the significant differences of the gene expression levels under a single pair of experiments (a control experiment and treatment experiment). In order to take into account the stochastic aspects of gene expressions, we model our algorithm on the t-test. In our approach and the t-test, the SD estimates of the gene expression levels are a criterion for the statistical judgment, but one of the key differences is how to estimate the SD for each gene.In the t-test, the SD estimates are derived from the same data set as those to be judged by the t-test, itself. This fact can be closely connected with the above-mentioned problem of replication. In our approach, the SD estimates, referred to here as a priori SD, are obtained from experimental results which are different from the target data set of the judgment.Statistics tells that the variability in the estimates of SD obeys the chi-squares distribution and is much larger than the variability in the estimates of averages, as long as the estimates are obtained by repetition. That is, the estimates of averages are more reliable. The t-test uses the SD estimates directly, but in this paper, the a prior SD is given as a function of the average of the gene expression levels. Then, we can easily expect that our approach can provide more stable judgment, but needs a sound model for the a prior SD.The idea of the a priori SD is not novel in the area of analytical chemistry. Since more than three decades ago, there have been published many theories and methods for estimating SD with no recourse to repetition, especially in instrumental analyses. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] In spite of varied symbols and terminology in the literature, the largest part of uncertainty equations proposed can take a universal form:where RSD denotes the relative standard deviation of measurements, sB denotes blank SD, A measurements (e.g., area), and I independent error. To our knowledge, Huber et al. A traditional method for comparing two expression levels of genes in microarray experiments is the two-sample t-test. Because of the difficulty in using a large number of microarrays, an alternative method is required which can provide a relia...