Bob Zeidman, a prominent forensic software engineer, found in 2012 that Microsoft’s early versions of MS-DOS did not contain any CP/M code. Microsoft then released DOS source in 2014. Zeidman decided to look again and will announce his findings at Vintage Computer Festival West. Discuss it here.
If you look at the first lines of DR-DOS you will find a 1976 copyright. I believe this easily can be explained that Dr-Dos contains CP/M code. I believe its a long story.
The research was about MS-DOS, not DR-DOS.