On July 31, 2012, the NBME retired Form 3 of the Step 1 NBME Comprehensive Basic Science practice exams. Form 3 is being replaced by Form 13 which will be more representative of the vignette format currently being used on Step 1. The question vignettes are much longer on forms 11, 12, and 13 and more similar in length to the current Step 1. However, it maytake more than one CBSE to get a good grasp of subject coverage and your performance in all subject areas.
There are currently 6 forms, although the NBME NSAS Program Guide reports there are 7 forms. This is apparently an error.
The CBSE Forms for Step 1 are: 5, 6, 7, 11,12, 13 - cost $50/exam
The CBSE Forms with comprehensive feedback are: 6, 7, 11, 12, 13 (all but Form 5) – cost $60/exam