Apparently the gene locus codes for individual variations in the level of synthetic activity on the part of the cell's "Golgi Apparatus," endoplasmic reticulum, and levels of messenger RNA. These three organs control the level of secretory activity as per the cell. Enzyme activity is , per se, highly associated with nuclear DNA (which of course is where the gene is). This apparently is the result of a phenomena called "co-adjacent alleles" where one gene locus seems to influence the repression or de-repression of another, geographically associated gene of otherwise unreleated nature.

The reason that it is apparently both ABO and secretor determined, rather than just one or the other, is that for example, an O non-secretor has more than an O secretor, but an O secretor has more than an A non-secretor, who has more than an A secretor.