Many enzymes involved in glycolysis such as GFP or Pgk1 do not cause any harm until they are overexpressed up to or close to the protein burden limit, which corresponds to 15% of the total proteins in the cell. Proteins that are toxic before reaching this limit cause harm via mechanisms other than the exhaustion of cellular resources. For example, while Tpi1 can still be expressed at relatively high levels (close to 15%), it causes protein aggregation. Enzymes such as Pfk1 or Adh3 can only be expressed at lower levels before they are toxic: Pfk1 causes too much catalytic activity while Adh3 overloads transport systems. Some proteins, for example Glk1, Pyk2 and Pdc1, are not harmful when overexpressed because they simply cannot reach the protein burden limit. Expression of these genes is lower because they use rare codons (sequences that are less optimal for protein production).