The Nov 2010 version of the PREX database contains 6325 total sequences from GenBank(nr), each categorized into one of six Prx subfamilies. Prx subfamily members were identified by DASP using a p-value cutoff of 10-9.
The Jan 2008 version of the PREX database contains 3516 total sequences from GenBank(nr), each categorized into one of six Prx subfamilies. Prx subfamily members were identified by DASP using a p-value cutoff of 10-8.
Information about the specific proteins used to determine protein subfamilies in each version is available here.
More detailed information about the methods used to create PREX is also available from Soito et al and Nelson et al.