For each transcript, we fit a two-way (12 X 2) general linear model using factors: age, strain and their interaction. The first factor has 11 levels starting from E12 to P9 time points . The second factor has two levels: DBA2 and BL6. p-values are calculated for the main and interaction effects. We are most interested in identifying genes for which there is a significant interaction between development stage and strain effect indicating that the strain alters the time course of gene expression.