Aim A prospective typing project to confirm HLA alleles with an unconfirmed status in the IMGT/HLA database identified sequence errors for 2 HLA-C alleles, C∗03:12 and C∗15:20. C∗03:12 was described in November 1999 on a reference cell from a Be The Match Registry volunteer. C∗15:20 was described in February 2007 on a reference cell typed through a NMDP prospective high resolution typing project. Methods A sample initially reported as C∗03:12 was re-typed by two independent labs, who both confirmed the presence of C∗03:03. Because of the discrepancy, the IMGT/HLA reference cell was evaluated at an NMDP contract lab by SBT to confirm the presence of C∗03:12. Concurrently, the IMGT/HLA reference cell carrying C∗15:20 re-typed as C∗15:27. Since the reference cell sequence contained a likely error, it was sent for cloning and sequencing of the putative C∗15:20 in isolation. A second recently recruited sample with C∗15:20 was identified and re-typed by SBT. Results The IMGT/HLA reference cell reported to carry C∗03:12 was corrected to C∗03:19, whereas the other sample with C∗03:12 retyped as the common C∗03:03, indicating two discreet typing errors. C∗03:12 differs from C∗03:19 at nucleotide (nt) 343 (A to G) at the end of exon 2. C∗03:12 and C∗03:19 are further differentiated from C∗03:03:01 at nt 419 and nt 420 in exon 3. C∗03:19 was described in February of 2005 and has a confirmed status in the IMGT/HLA database. Both the IMGT/HLA reference cell and the other sample with C∗15:20 retyped as C∗15:27. C∗15:20 differs from C∗15:27 at nt 259 (G to A) and nt 261 (G to C) in exon 2. C∗15:27 was described in January 2010 and has a confirmed status in the IMGT/HLA database. Conclusions As a result of the resequencing results of the reference cells, which were found to contain errors, C∗03:12 and C∗15:20 will be deleted from the IMGT/HLA database. This project highlights the importance of confirming IMGT/HLA database allele sequences to validate existence of reported alleles.