The Gene Expression Database (GXD) collects and integrates the gene expression information in MGI. GXD's primary emphasis is on endogenous gene expression during mouse development. Data are acquired from the literature by our curatorial staff and via direct submission from large-scale data providers, as well as individual labs. New expression data are made available on a daily basis. GXD currently collects detailed expression data from the following assay types: RNA in situ hybridization; Immunohistochemistry; in situ reporter (knock in); RT-PCR; Northern and Western blots; and RNase and Nuclease S1 protection assays It is our curatorial policy to annotate expression patterns based solely on what is written in the manuscript; we do not interpret images presented within papers. We are happy to accept more detailed descriptions of expression patterns from authors and will update database entries accordingly. GXD focuses on endogenous gene expression data in wild-type and mutant mice, including reporter data from knock-in experiments. Currently GXD does not include expression data from other types of transgenic animals, conditional knock-outs, chimeras, animals subjected to treatments, or from cultured material.