Reproduction and fertility rely on endocrine hormones. Treatments for endocrine dysfunction involve pharmacological agents to regulate hormone production or action, but many involve off-target effects, the need for long-term use and close clinical monitoring. We aimed to develop novel gene delivery systems that specifically target endocrine cells to safely and efficiently modulate endogenous in vivo hormone production. Our team has developed nanoparticle-based, endocrine-targeting technologies and validated them in mice. We have validated the following components: 1) An ionisable lipid nanoparticle sphere that encapsulates a DNA payload. The nanoparticles are made from FDA-approved excipients and are low-immunogenic and of a customisable, uniform size. They are rapidly synthesised on a platform that can be scaled for clinical translation. They are administered via intraperitoneal injection and are highly stable in circulation. Once the nanoparticles reach their target cell, they cross the plasma membrane, and the DNA payload is released. The nanoparticles can carry large (up to 14 kb) genetic payloads. 2) The nanoparticle surface can be coated with targeting peptides that bind to an extracellular receptor on the target cell to customise cell-specificity. 3) The DNA payload can be further customised for specificity by the inclusion of a cell-specific promoter. 4) The DNA payload can be customised to drive (cDNA) or suppress (shRNA, miRNA) gene expression in the target cell. Finally, we have administered this system to mice to target the adrenal and testis and have demonstrated its ability to drive changes in hormone production. We conclude this customisable system can be used to rapidly generate transgenic models from adult wildtype mice within 4-8 weeks, with an 85-90% cost reduction compared to traditional transgenic lines, and with significant advantages for animal welfare. This technology offers a myriad of opportunities to regulate or treat endocrine and reproductive disorders with a clear clinical needs-gap.