Cytochrome P450 17α-hydroxylase, 17,20-lyase (P450c17) is a key enzyme in the synthesis of cortisol in the zona fascicula of the adrenal cortex, and the synthesis of androgen precursors in the adrenal zona reticularis and the gonads. Each of these reactions require electrons transferred by the electron donor cytochrome P450 oxidoreductase. The 17α-hydroxylation of its substrate occurs in all cells where P450c17 is expressed. Remarkably, a second, subsequent reaction, namely the 17,20-lyase activity, only occurs in the zona reticularis and gonads. The specificity of the second reaction is due to the interaction with the haem-protein cytochrome b5. Surprisingly, cytochrome b5 and cytochrome P450 oxidoreductase have overlapping sites of interaction on the surface of P450c17. This poses the question as to how cytochrome b5 and cytochrome P450 oxidoreductase interact with P450c17 structurally, functionally and physiologically? This conundrum can be resolved based on the observation that P450c17 can homo-dimerise. A homodimer would allow cytochrome P450 oxidoreductase to bind to one P450c17 monomer of the P450c17 homodimer whilst cytochrome b5 could bind to the other P450c17 monomer simultaneously at the surfaces distal to the dimer interface. This structure is likely to be dynamic in vivo. Our modelling predicts that the proteins can assemble as a stable tetramer and is fully consistent with extensive experimental data that have been published over the last two decades. Predictions derived from this structural model are currently being tested by a range of in vitro and in vivo experimental approaches.