# Customize how the reference numbers appear in the bibliography when using biblatex?

I wish to customize the font for when printing the reference numbers (the to the left of all references, commonly surrounded by brackets) in the biblatex package. I hope that there is some command that you can simply renew, but I'm unable to find it.

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Redefine the labelnumberwidth field format. (In my exampe I add bold formatting.)

\documentclass{article}

\usepackage{biblatex}

\DeclareFieldFormat{labelnumberwidth}{\textbf{\mkbibbrackets{#1}}}

\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@misc{A01,
author = {Author, A.},
year = {2001},
title = {Alpha},
}
\end{filecontents}