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Background: The commonly used laboratory rat, Rattus norvegicus, is unique in having multiple Sry gene copies found on the Y chromosome, with different copies encoding amino acid variations that influence the resulting protein function. It is not clear which Sry genes are expressed at the onset of testis differentiation or how their expression correlates with that of other genes in testis-determination pathways., Methods: Here, two independent E11-E14 developmental RNAseq datasets show that multiple Sry genes are expressed at E12-E13., Results: The identified copies expressed during testis initiation include Sry4A, Sry1, and Sry3C, which are conserved in every strain of Rattus norvegicus with genomes sequenced to date., Conclusions: This work represents a first step in defining the complex environment of rat testis differentiation that can open the door for generating sex reversal model systems using embryo manipulation techniques that have been available in the mouse but not the rat.