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docs/tutorials/004_compare_specificity.md

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 # Tutorial: Compare Specificity
 
+Tissue specificity can be combined with gene families to pick up genes with conserved expression between species but
+also to detect genes within one species that sub- or neo-functionalized after duplication. Look at the examples below to
+see how.
+
+
 ## Conserved Expression
 
+Some low-abudant genes can cause false positives when looking for tissue/condition specificity. These adverse results can
+be greatly reduced by looking for genes that are specific in a certain tissue in one species and have an ortholog 
+(or homolog) specifically for the corresponding tissue is another species.
+
+In this example *Arabidopsis thaliana* silique specific genes are matched with their orthologs expressed specifically in
+Tomato (*Solanum lycopersicum*) fruis. This feature is available from the tools menu below the button 
+**Compare specificity**. The SPM (Specificity Metric) slider for tomato is adjusted to yield a similar number of hits
+as in *Arabidopsis thaliana*. 
+
+![Compare specificity](images/compare_specificity_entry.png "Compare specificity entry") 
+
+In the resulting page orthogroups with a member expressed in one, the other or both selected conditions are shown in a
+venn diagram, with a detailed list below. By hovering over a gene, the description can be observed. 
+
+![Compare specificity](images/compare_specificity_result.png "Compare specificity result") 
+
+Here, the first reported hit is AGL1 aka. SHATTERPROOF 1, a known fruit development gene, clearly illustrating the power of this 
+approach.
+
 ## Sub- or Neofunctionalization