--- a/genetrack.xml	Wed Dec 16 12:43:22 2015 -0500
+++ b/genetrack.xml	Wed Dec 16 12:43:31 2015 -0500
@@ -40,7 +40,7 @@
         <param name="exclusion" type="integer" value="20" min="1" label="Peak exclusion zone" help="Exclusion zone around each peak that prevents others from being called." />
         <param name="up_width" type="integer" value="10" min="0" label="Exclusion zone of upstream called peaks" />
         <param name="down_width" type="integer" value="10" min="0" label="Exclusion zone of downstream called peaks" />
-        <param name="filter" type="integer" value="3" min="0" label="Absolute read filter" help="Removes peaks with lower peak height." />
+        <param name="filter" type="integer" value="1" min="0" label="Absolute read filter" help="Removes peaks with lower peak height." />
     </inputs>
     <outputs>
         <collection name="genetrack_output" type="list" label="Genetrack results on ${on_string}">
@@ -88,10 +88,8 @@
     <help>
 **What it does**
 
-<![CDATA[
-
-GeneTrack separately identifies peaks on the forward "+” and reverse “-” strand.  The way that GeneTrack works
-is to replace each tag with a probabilistic distribution of occurrences for that tag at and around its mapped
+GeneTrack separately identifies peaks on the forward "+” (W) and reverse “-” (C) strand.  The way that GeneTrack
+works is to replace each tag with a probabilistic distribution of occurrences for that tag at and around its mapped
 genomic coordinate.  The distance decay of the probabilistic distribution is set by adjusting the value of the
 tool's **Sigma to use when smoothing reads** parameter.  GeneTrack then sums the distribution over all mapped
 tags.  This results in a smooth continuous trace that can be globally broadened or tightened by adjusting the
@@ -101,9 +99,34 @@
 within that exclusion zone.  In rare cases, it may be desirable to set different exclusion zones upstream (more 5’)
 versus downstream (more 3’) of the peak.
 
-]]>
+.. image:: $PATH_TO_IMAGES/genetrack.png
+
+GeneTrack continues through the data in order of peak height, until no other peaks are found, and in principle will
+call a peak at a single isolated tag, if no filter is set using the tool's **Absolute read filter** parameter.  A 
+filter value of 1 means that it will stop calling peaks when the tag count in the peak hits 1 (so single tag peaks
+will be excluded in this case).  GeneTrack outputs **chrom** (chromosome number), **strand** (+/W or -/C strand),
+**start** (lower coordinate of exclusion zone), **end** (higher coordinate of exclusion zone), and **value** (peak
+height).  Genetrack's GFF output reports the start (lower coordinate) and end (higher coordinate) of the exclusion
+zone.
+
+In principle, the width of the exclusion zone may be as large as the DNA region occupied by the native protein plus
+a steric exclusion zone between the protein and the exonuclease.  On the other hand the site might be considerably
+smaller if the protein is in a denatured state during exonuclease digestion (since it is pre-treated with SDS).
 
-.. image:: $PATH_TO_IMAGES/genetrack.png
+In general, higher resolution data or smaller binding site size data should use smaller sigma values. Large binding
+site size data such as 147 bp nucleosomal DNA use a larger sigma value like 20 (-s 20).  For transcription factors
+mapped by ChIP-exo, sigma may initially be set at 5, and the exclusion zone set at 20 (-s 5 –e 20).  Sigma is typically
+varied between ~3 and ~20. Too high of a sigma value may merge two independent nearby binding events.  This may be
+desirable if closely bound factors are not distinguishable.  Too low of a sigma value will cause some tags that
+contribute to a binding event to be excluded, because they may not be located sufficiently close to the main peak.
+If alternative (mutually exclusive) binding is expected for two overlapping sites, and these sites are to be
+independently recorded, then an empirically determined smaller exclusion zone width is set.  Thus the value of sigma
+is set empirically for each mapped factor, depending upon the resolution and binding site size of the binding event.
+
+It might make sense to exclude peaks that have only a single tag, where -F 1 is used, or have their tags located on
+only a single coordinate (called Singletons, where stddev=0 in the output file).  However, low coverage datasets might
+be improved by including them, if additional analysis (e.g., motif discovery) validates them. In addition, idealized
+action of the exonuclease in ChIP-exo might place all tags for a peak on a single coordinate.
 
 -----