In the field of population genetics there are many measures of genetic diversity and population differentiation. The best known is Wright’s Fst, later expanded by Cockerham and Weir, which is very widely used as a measure of separation between populations. More recently a multitude of other measures have been developed, from Gst to D, all with different features and disadvantages. One thing these measures all have in common is that they are symmetric, which is to say that the Fst between population A and population B is the same as that between population B and population A. Following my work on GenePlot, a visualization tool for genetic assignment, I am now working on the development of an asymmetric measure, where the fit of A into B may not be the same as the fit of B into A. This measure will enable the detection of scenarios such as “subsetting”, the relationship between a large, diverse population A and a smaller population B that has experienced genetic drift since being separated from A. The measure has several features that distinguish it from existing measures, and is constructed using the same saddlepoint approximation method underlying GenePlot, and which is used to approximate the multi-locus genetic distributions of populations.