The dual enzyme activities were for many decades assumed to represent two entirely different genes and enzymes. Thus, medical textbooks and nosologies until quite recently described two different diseases: "17α-hydroxylase deficient CAH", and a distinct and even rarer defect of sex steroid synthesis termed "17,20-lyase deficiency" (which is not a form of CAH). In the last decade it has become clearer that the two diseases are different forms of defects of the same gene. However, the clinical features of the two types of impairment are distinct enough that they are described separately in the following sections.