The T2 bundle provides a variety of separate support functions,
for using Cyrillic characters in LaTeX: - the mathtext package,
for using Cyrillic letters 'transparently' in formulae - the
citehack package, for using Cyrillic (or indeed any non-ascii)
characters in citation keys; - support for Cyrillic in BibTeX; -
support for Cyrillic in Makeindex; and - various items of font
support.