The typographical corrections below were made to the article on the date noted. These changes have been incorporated in the article and the details are documented here.

28 June 2001:

In the Methods section, the subheading "Confocal microscopy" was corrected to "Confocal and fluorescence light microscopy".

In the Methods section, the following sentence was appended to the paragraph under the subheading "Confocal and fluorescence light microscopy" (formerly "Confocal microscopy"):

The conventional fluorescence images were taken with a Zeiss Axioplan 2 microscope.