The study investigates the wavelength mutations and posttranslational autoxidation of green fluorescent protein (GFP) from the jellyfish *Aequorea victoria*. GFP is a unique protein with strong visible absorbance and fluorescence due to a p-hydroxybenzylidene-imidazolidinone chromophore, which forms through the cyclization and oxidation of a Ser-Tyr-Gly sequence at positions 65–67. The researchers found that the formation of the final fluorophore requires molecular oxygen and proceeds independently of dilution, suggesting that the oxidation does not require enzymes or cofactors. They also report that mutagenesis of GFP led to variants with altered spectra, including a mutant that fluoresces blue and contains histidine instead of tyrosine at position 66. These findings extend the utility of GFP in molecular and cell biology by enabling in vivo visualization of differential gene expression and protein localization, as well as measuring protein association through fluorescence resonance energy transfer. The study provides insights into the mechanism of fluorophore formation and the potential for tailoring GFP's fluorescence properties.The study investigates the wavelength mutations and posttranslational autoxidation of green fluorescent protein (GFP) from the jellyfish *Aequorea victoria*. GFP is a unique protein with strong visible absorbance and fluorescence due to a p-hydroxybenzylidene-imidazolidinone chromophore, which forms through the cyclization and oxidation of a Ser-Tyr-Gly sequence at positions 65–67. The researchers found that the formation of the final fluorophore requires molecular oxygen and proceeds independently of dilution, suggesting that the oxidation does not require enzymes or cofactors. They also report that mutagenesis of GFP led to variants with altered spectra, including a mutant that fluoresces blue and contains histidine instead of tyrosine at position 66. These findings extend the utility of GFP in molecular and cell biology by enabling in vivo visualization of differential gene expression and protein localization, as well as measuring protein association through fluorescence resonance energy transfer. The study provides insights into the mechanism of fluorophore formation and the potential for tailoring GFP's fluorescence properties.