Genotype data missing in some individuals

Dear all, I’ve analyzed a tacrolimus PopPK in pediatric patients. As you know, CYP3A5 genotype can change the tacrolimus PK significantly, 3A5 genotyping was performed in the study, however, in 20% of the subjects, the genotype data was missed. Then, how can I reflect the CYP3A5 genotype effect to the tacrolimus population model appropriately? Is there any solution? Best regards, SoJeong Yi