However, sometimes the proteasome breaks down proteins without the help of ubiquitin tags, leading researchers to suspect that there was another, ubiquitin-independent mechanism of protein degradation.

Source: https://phys.org/news/2023-08-scientists-previously-unknown-cells-proteins.html

UFMylation is an enigmatic QC pathway based on a protein post-translational modification that resembles ubiquitin and its functions are still being deciphered.

Source: https://phys.org/news/2023-02-autophagy-molecular-self-eating.html