I have RNAseq data for human subjects and plan to do a differential gene expression analysis using DESeq2. The covariates I plan to use include sex, age, genotypic principal components, and substance use. However, I have missing data for principal components and substance use and would like to know how to code these missing data in the SampleTable. Can I put "." for missing data or would I have to take out the subjects with missing data? Thanks.