Anti-Jk3 is a rare alloantibody to a high-prevalence antigen primarily seen in individuals of Polynesian descent and is associated with a handful of well-established variants of the SLC14A1 gene. We report a case of the Jknull phenotype, associated with formation of anti-Jk3, in a patient of non-Polynesian descent. This patient, a 51-year-old woman self-described as of Jamaican and Scottish ancestry, presented to our hospital for oncologic care. The patient’s blood sample typed as blood group A, D+. All screening and panel reagent red blood cells showed reactivity, ranging from 2 to 4+; autocontrol and direct antiglobulin test were both negative. Antigen phenotyping revealed Jk(a–b–), leading to suspicion for anti-Jk3, which was subsequently confirmed by our immunohematology reference laboratory. Given her reported familial background, testing of the SLC14A1 gene was performed, revealing that the patient was heterozygous for the single nucleotide variant (SNV) at c.838G>A in exon 8 and therefore carries both JK*01 and JK*02 alleles that encode Jka and Jkb, respectively. However, the patient was found to be heterozygous for several additional SNVs: c.28G>A in exon 3; c.191G>A, c.226G>A, and c.303G>A in exon 4; and c.757T>C in exon 7. The patient’s Jk(b–) phenotype can be explained by coinheritance of c.838A with c.191G>A, which defines null allele JK*02N.09. Coinheritance of SNVs c.28G>A and c.838G with rare SNV c.757C that is predicted to cause a non-conservative amino acid change (p.S253P) likely accounts for the complete serologic absence of Jka and the ability to form anti-Jk3 in this case. This finding would represent a new JK*01 null allele. This evaluation illustrates the importance of genetic analysis in identifying the factors preventing a high-prevalence antigen from being expressed, particularly when discovered outside of an expected racial or ethnic group.