Fig. 4. Real-time RT-PCR analysis of markers involved in the sex determination process. Sox9 expression (shown on the X axis) in individual E13.5 knock-out gonads was variable with levels in some gonads being comparable to those in female mice. (A) Mis expression was directly dependent on Sox9 levels indicating direct activation of its promoter (correlation coefficient r=0.9905; P<0.0001). Note that Mis expression in mice with low levels of Sox9 was still higher than that in control females. (B) Expression of Sox8 also showed a clear dependence on Sox9 levels (r=0.8444; P<0.0001). (C) No such direct relationship for the steroidogenic factor Sf1 was observed (r=0.3173; P=0.2689). (D) Gonads with very low levels of Sox9 showed persistent expression of the Sry gene.