BACKGROUND-The partial D phenotype DIIIa was originally reported to be associated with 455A>C in Exon 3, 602C>G in Exon 4, and 667T>G in Exon 5. Other alleles with these changes were subsequently identified and designated DIII Types 5, 6, and 7, as they had additional alterations. The observation that DNA samples associated with the DIIIa phenotype had more changes than those originally reported motivated us to reanalyze the DIIIa probands (BP and DJ) from the original study. We also studied additional DIIIa samples to clarify the RHD background and establish the associated RHCE.

3159 Poster Board III-96 Background According to the original (and only) report, the low prevalence Rh antigen, STEM, is associated with an altered e phenotype. Approximately 65% of hrS– and 30% of hrB– RBCs from South African donors are STEM+. STEM has a variable expression, which is an inherited characteristic. Anti-STEM has induced mild HDFN (Marais, et al., Transf Med 1993;3:35-41). The purpose of this study was to determine the molecular basis associated with STEM expression. Material and Methods Blood samples and reagents were from our collections. Hemagglutination and DNA extraction were performed by standard methods. Molecular testing included direct sequencing and cloning of cDNA, AS-PCR, PCR-FRLP, and sequencing specific exons of gDNA. Results Three STEM+ samples (including the original index case) had RHCE*ceBI [ce 48C (16C), 712G (238V), 818T (273V), 1132G (378V)] (Noizat-Pirenne, et al., Blood 2002;100:4223-31) and 6 had a new allele, which we name RHCE*ceSM (ce 48C, 712G, 818T). In contrast, 8 STEM– samples (which included hrS– and hrB– samples) did not have the RHCE*818C>T change. RBCs with the ceBI phenotype expressed STEM more strongly than those with the ceSM phenotype. Conclusions The previously reported allele RHCE*ceBI and a new allele, RHCE*ceSM, encode the STEM antigen. This study also revealed other new findings: (i) ceSM encodes a weaker expression of STEM than does ceBI, which explains the previously reported variable expression, (ii) provides an explanation for why not all hrS– and hrB– RBCs express STEM. RBCs with ceAR, ceMO, and ceEK, phenotypes are hrS– STEM–, and RBCs with ceS phenotypes type hrB–, STEM–, (iii) ceBI and ceSM do not express hrS but do express hrB. It is likely that anti-hrS made by hrS– STEM– people (ceAR, ceMO, ceEK) will be incompatible with hrS– STEM+ RBCs, and vice versa. Our findings provide a means to positively identify the STEM+ phenotypes, which, by hemagglutination, is virtually impossible due to the dearth of anti-STEM. Further, it provides a tool to provide suitable antigen-negative RBC products to a patient who has made an ‘e-like’ antibody. Disclosures No relevant conflicts of interest to declare.