<b>Bright and stable monomeric fluorescent protein derived from StayGold</b>

High brightness and photostability of StayGold make it a particularly attractive probe for long-term live cell imaging. However, its dimeric nature precludes its application as a fluorescent tag for some proteins. Here, we report the development and X-ray structures of a monomeric variant of StayGold (mBaoJin), which preserves the beneficial properties of its precursor while serving as a tag for structural proteins and membranes. We compare mBaoJin to other state-of-art GFPs and utilize it for super-resolution long-term live cell imaging and expansion microscopy.