“…Azam et al [14 ] developed a reduced‐order sliding mode observer to estimate the unknown state of the MDM2–p53 protein complex by enforcing a sliding mode on the error in the concentration of p53 protein. Aguilar‐Lopez and Mata‐Machuca [15 ] developed a state observer to estimate the time evolutions of three protein concentrations in a gene expression model by measuring one mRNA concentration using the RT–PCR method. Cacace et al [16 ] developed a Luenberger observer which estimated the concentration of mRNA and the concentration of protein with minor parameter uncertainties.…”