Disclosed herein is a method of performing polymerase chain reaction (PCR) to determine a repeating number of CGG sequence in fragile X mental retardation 1 (FMR1) gene. Also disclosed herein are a kit, and uses thereof in making a diagnosis of Fragile X syndrome (FXS) in a human subject based on the repeating number of the CGG sequence in a DNA sample isolated from the human subject. According to embodiments of the present disclosure, the kit comprises four primers, in which the first primer comprises a first polynucleotide sequence of SEQ ID NO: 1; the second primer comprises a second polynucleotide sequence of SEQ ID NO: 2; the third primer comprises a third polynucleotide sequence of SEQ ID NO: 3, and a non-human sequence disposed at and connected to the 5′-end of the third polynucleotide sequence; and the fourth primer comprises the non-human sequence.