The cycle number (nc) of a recycling receptor is defined as the average number of round trips (cell surface–endosome–cell surface) the receptor can make before it is degraded. This characteristic parameter of recycling receptors can be easily determined from the receptor’s half-life (t½, the time in which 50% of the receptor is degraded) and cycling time (Tc, the time a receptor needs to complete a round trip). Relationship analyses revealed that nc increases linearly with increasing t½ and decreases exponentially with increasing Tc. For commonly observed t½ and Tc values, it was calculated that recycling receptors have nc values of <300. In addition, it was found that recycling receptors in cancer cells have generally smaller nc values (<100), whereas recycling receptors in normal cells have larger nc values (>100). Based on this latter finding, the cycle number nc may be a useful criterion for distinguishing between cancer and normal cells.