Gene Length Variable ** Use this field to select a variable indicating the length of gene (or exon, if the input data is at the exon level). To Specify the Gene or LocusLink ID Variable: Specify the Input SAS Data Set . All of the variables in the specified data set are displayed in the Available Variables field. Examine the list of available variables. Highlight the variable that lists the length of each of the genes. Click to add the highlighted variable to the Gene Length Variable field, as shown below: