a) Replicate measurements of tyrosine phosphorylation by SCX; in the first bar is the combined result of three independent SCX experiments ( n=2177), in the second bar is the first trypsin experiment ( n=1331), in the third bar is the Lys-N experiment ( n=881) and in the fourth bar is an additional trypsin repeat experiment ( n=123); n=total number of unique phosphosites (S, T, Y). ( b) Compared with other organisms Trichoplax exhibits a high percentage (3.9%) of tyrosine amino acids in its genome (red bars). The percentage of detected tyrosine phosphosites in Trichoplax phosphoproteomics data sets is four- to fivefold higher than detected in large-scale phosphoproteomics data sets for other organisms, including H. sapiens , 14, 35 M. musculus , 13 D. melanogaster and 11 S. cerevisiae (blue bars, 36 Supplementary Data 4). Only species for which comprehensive protein phosphorylation data is also available, obtained using alike protocols, are shown . ( 37, 38, 39, 40, 41 c) Trichoplax contains a relative high number of readers (SH2 phosphotyrosine recognition domains) and erasers (tyrosine phosphatases), compared with writers (tyrosine kinases) involved in tyrosine signalling. Tyrosine-kinase domains, SH2 domains and protein tyrosine phosphatase domains were detected using HMM models from SMART using the online SMART tool ( 28 http://smart.embl-heidelberg.de/). Inset shows phosphotyrosine signalling as a tripartite system comprising tyrosine kinase (writer), tyrosine phosphatase (eraser) and SH2 domain (reader).