The development pathway of the male gonad, the testis, occurs in response to the expression of the Y chromosome-linked Sry gene within the somatic cells of the primordial gonad. In mice, the expression of Sry is tightly spatially and temporally regulated, with testis development only inducible within a brief time window during embryonic development. Disruption or delay of Sry expression can result in complete XY sex reversal, giving rise an ovary, or an intermediate gonadal phenotype, producing an ovotestis. A mouse strain with a reported anomaly in male sex development is the A/HeJ mouse, first reported in 2008 by Hunt et al in “The mouse A/HeJ Y chromosome: Another good Y gone bad”. This mouse strain exhibits a disturbance in the male sex determination pathway, with 4% of non-productive males presenting as overt hermaphrodites, and a further 17% having small testes with no epididymal sperm. It was hypothesised that there is a deletion at or near the Y chromosome centromere which affects the expression of Sry during embryogenesis. This study examines the region surrounding the Y chromosome centromere of A/HeJ and other closely related strains. Epigenetic and gene expression analyses have been performed on Sry in this strain to determine the aetiology of this sex development disturbance.