CTLA4 and type 1 diabetes mellitus: Explicitly incorporating sex as a core factor in study design and clinical practice, by testing sex‐biassed penetrance of GWAS loci and established T1D genes such as HLA, INS VNTR, IL2RA, FOXP3, CTLA4 and CLEC16A, integrating sex‐modified immune signatures, and modelling endocrine and chromosomal effects, will ensure that mechanistic insights are translated into tailored strategies for prediction, monitoring and prevention in both boys and girls at risk.