Figure 1. Co-existence lines with lower and upper critical solution temperatures.

A) Phase separation of biopolymers can be observed macroscopically using differential interference contrast (DIC) microscopy and is manifest as the formation of droplets. B) Biopolymers in solution can be in the one-phase or two-phase regime; in the latter, a dense and a light phase coexist. Whether the phase transitions have an upper or lower critical solution temperature (UCST or LCST, respectively) is defined by their critical temperatures for mixing; if the protein mixes when the temperature is lowered, the phase transition has a LCST; if mixing occurs at increasing temperature, the phase transition has a UCST.