By assessing DNA methylation upstream to the CTG expansion in motile sperm cells of four DM1 patients, together with availability of human embryonic stem cell (hESCs) lines with paternally inherited hypermethylated expansions, we exclude the possibility that DMPK hypermethylation leads to selection against viable sperm cells (as indicated by motility) in DM1 patients. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.