Marker Physical Position Use this field to specify the variable listing the physical positions (base pairs) of the markers. To Specify the Marker Physical Position Variable: Specify the Annotation SAS Data Set. All of the variables in the specified annotation data set are displayed in the Available Variables field. Examine the list of available variables. Highlight the variable whose values indicate the physical position of the markers on the chromosomes. Click to add the highlighted variable to the Marker Physical Position field, as shown below: