Jan met the Frank family through his fiancée, Miep Santrouschitz. From 1936 onwards, he was frequently present at Saturday afternoons, when the Franks invited friends and acquaintances.
When Jews were no longer allowed to own businesses, Otto Frank was grateful for Jan’s help. Together with Victor Kugler, Jan founded the company Gies & Co. to take over Otto's company Pectacon, and Jan took on the role of supervisory director. This was a way to keep Otto's business safe from the Nazis, because with a Jewish owner (Otto Frank) it would have been placed under German supervision.