APOE and diabetes mellitus: The main reason may be because (1) there were racial differences in the distribution of ApoE alleles, (2) different methods used to detect ApoE genotype may affect the accuracy of the results [31], (3) there are bias and confusion bias in patients selection and statistical processing such as small sample size and short duration of diabetes, (4) observation period was too short in some prospective study to observe the DN occurrence; some patients in the control group may have DN some years later, and (5) different studies had different DN diagnosed standards.