TetR regulating peptides are mostly composed of 12 to 16 amino acids and can exert highly diverse effects such as induction, anti-induction and co-repression. The regulatory flexibility displayed by these peptides, together with the lack of small molecule antagonists, led us to attempt to isolate regulatory peptides for the efficient and most widely used transregulator rtTA2S-M2. During the experimental procedure, a peptide was discovered that acted as a dox antagonist in both bacteria and a human cell line. Extensive analysis of the peptide revealed residues which are necessary for its antagonistic activity and shed light on its potential binding site on rtTA. We could further show that expression of the peptide in a human cell line led to a significantly faster and stronger decline in rtTA-mediated activation of gene expression compared to samples in which dox was removed by medium exchange. Therefore, this novel rtTA-regulating peptide represents the basis for developing small molecule antagonists that could complement the widely used Tet-On system with an efficient means to rapidly switch off rtTAmediated activation of target gene expression at will. Compounds that can mimic and block natural hormones and cause adverse health effects in humans and wildlife are referred to as endocrine-disrupting compounds. Studies have demonstrated that a wide range of EDCs can lead to serious problems, such as infertility. Cypermethrin, a type II synthetic pyrethroid insecticide, replaces traditional organochlorine and organophosphate pesticides and has been widely used. Different studies had indicated that CYP treatment decreases the layers of spermatogenic cells, increases the inside diameter of seminiferous tubules, decreases Star expression in adolescent mice, disturbs the array of spermatogenic cells, reduces sperm count and motility in male mice, decreases serum testosterone levels, and increases serum follicle-stimulating hormone and luteinizing hormone levels. It has been demonstrated that CYP exerts anti-androgen effects in androgen receptor reporter gene assays and can induce ER transactivity. Most studies have used higher doses of CYP, ranging from 39.66 mg/kg/day to 485 mg/kg/day and even toxicological doses, and most studies have focused on postnatal exposure. However, there are no reports describing the effects of lower dosage or environmental exposure levels during fetal exposure on the growth and development of testes. Various studies have investigated the effects of EDCs on the growth and development of the fetus, which is sensitive to hormonal fluctuations. Impaired reproductive development has been demonstrated in the sons of female gardeners or farmers where pesticides have been used. This study aimed to assess CYP exposure during the perinatal period to determine its effect on fetal development and its long-term impact on male reproduction in C57BL mice.