The early region of BK virus (BKV) is known to encode two well-characterized tumour (T) antigens, large T antigen (TAg) and small T antigen (tAg). In this study, we provide evidence of a third early BKV mRNA that codes for an additional early region product with an apparent molecular mass of 17-20 kDa. This truncated form of TAg (truncTAg) is expressed from an alternatively spliced mRNA that is derived from the excision of a second intron from the mRNA encoding TAg. The first 133 aa of truncTAg are identical to those of TAg but the additional splice results in translation from a different reading frame, adding three new amino acids before reaching a stop codon. TruncTAg is expressed in both BKV-transformed and lytically infected cells and it is found to be primarily localized to the nucleus. The function of BKV truncTAg is likely to be relevant to transformation, similar to the additional T antigens of simian virus 40, JC virus and mouse polyomavirus. INTRODUCTIONBK virus (BKV) was first isolated in 1971 from the urine of an immunocompromised renal transplant patient (Gardner et al., 1971). It is a member of the polyomavirus family and is ubiquitous in the human population (Knowles et al., 2003). Following primary infection in early childhood, BKV persists in the kidneys and, upon immunosuppression of the human host, causes significant morbidity, particularly in bone marrow and renal transplant patients (Acott & Hirsch, 2007;Bohl & Brennan, 2007;Dropulic & Jones, 2008;Pavlakis et al., 2006). Polyomavirus nephropathy due to BKV reactivation is an emerging challenge in renal transplant recipients, in whom lytic replication can lead to destruction of the transplanted organ. BKV also has tumorigenic capabilities, which have been well demonstrated in experimental models (Tognon et al., 2003). It transforms rodent cells in culture, induces kidney tumours in transgenic mice and causes the immortalization of human cells either alone or in the presence of additional oncogenes such as c-Ha-ras or adenovirus E1A (reviewed by Imperiale & Major, 2007). Evidence in support of BKV as a potential cofactor in human cancers has been reported and others have observed the presence of BKV in cancerous prostates (Balis et al., 2007;Das et al., 2004Das et al., , 2008Fioriti et al., 2007;Lau et al., 2007;Zambrano et al., 2002). Understanding the complete life cycle of the virus and how it is regulated in these different clinical situations will be critical in the development of possible therapeutic interventions.The BKV genome is divided into the early region, the late region and the non-coding control region, which has the origin of DNA replication as well as elements for transcriptional regulation of both the early and the late genes (Imperiale & Major, 2007). The early region encodes the large tumour (T) antigen (TAg) and the small tumour antigen (tAg), which are products of two alternatively spliced mRNAs and are the first proteins to be expressed during infection. The late region encodes the capsid proteins VP1, VP2 and VP3 and th...

authors request that the following be noted. Several errors have been identified in the IL-14 cDNA sequence shown in Fig. 1. The translation of this sequence does not predict an open reading frame that would result in the production of a 50-60-kDa protein. A reading frame in the 5Ј to 3Ј direction (plus strand) predicts a 7.7-kDa protein and a reading frame in the 3Ј to 5Ј direction (minus strand) predicts a 36.4-kDa protein. The relationship of this sequence to IL-14, if any, is uncertain. The corrected sequence is shown below.