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 being 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 to distinguish between cancer and normal cells.