Mass spectrometry (MS) was once a technique for physicists and chemists, but today it's increasingly important for biological studies. For example, during the 1990s, it was discovered that MS could be used to identify gelseparated proteins using much smaller samples than are required by other chemical techniques. Since then, further technological advances have established MS as the definitive tool for studying the primary structure of proteins and as a key technology in proteomics. A problem, though, is that biologists often do not have the background to critically interpret the results of more challenging MS experiments. So, on page 699, Hanno Steen and Matthias Mannexplain the principles of MS-based peptide sequencing that are important to appreciate and interpret the outcome of proteomics experiments. Furthermore, on page 763, Alisdair R. Fernie, Richard N. Trethewey, Arno J. Krotzky and Lothar Willmitzer describe another application of MS — in the measurement of metabolites for metabolite profiling.

Another important and influential discovery of the 1990s was the identification of aquaporins — molecular water channels. As described by Landon S. King, David Kozono and Peter Agre on page 687, recognition of the unique properties of the aquaporins led to a paradigm shift in our consideration of membrane permeability, and the Nobel Prize in Chemistry 2003 was awarded to Peter Agre in recognition of this discovery.

Finally, this month we celebrate our Series of articles on plant cell biology, which appeared throughout 2004, with a Web Focus (see http://www.nature.com/nrm/focus/plant_biology ).We conclude the Series for now with a Review on intercellular communication in plants by William J. Lucas and Jung-Youn Lee (see page 712), and all the articles from the Series will be freely accessible online throughout September.