Sir

Readers of J. A. M. Graves's News & Views article 'Sex determination: Birds do it with a Z gene' (Nature 461, 177–178; 2009) may be left with the impression that a particular gene, DMRT1 on the Z chromosome, is the master determinant of sex in birds. However, the case is far from closed.

Graves discusses the findings of Craig A. Smith and colleagues (Nature 461, 267–271; 2009). These data support the view that DMRT1 is required for testis development in chickens. But as the authors state, “Although Z-linked DMRT1 is required for testis development, it is possible that another Z-linked gene lies upstream of this gene in the avian male-determining pathway ... It is also possible that a female determinant lies on the avian W sex chromosome”.

It may turn out that DMRT1 does determine sex in birds. But as yet other possibilities remain open.