Three necessary and important diagnostic criteria of CPP were used by all included studies: firstly, early appearance of secondary sexual characteristics in girls before 8 years (boys before 9 years); secondly, GnRH stimulation test indicating LH >12 IU/L (LH > 25 IU/L in boys), and the ratio of LH/FSH between 0.6 to 1.0; lastly, enlarged gonads with pelvic ultrasound showing increased ovarian volume >1 ml and several ovarian follicle with diameter > 4 mm (enlarged testicular volume ≥ 4 ml in boys). Here, GNRH1 is linked to central precocious puberty.