HHLA1 and teratocarcinoma: Furthermore, we can explain the prostate-specific and ERG-negative expression pattern of the SLC45A3-ELK4 fusion based on expression of the 5' gene SLC45A3. Similarly, previous researchers observed the HHLA1-OC1 fusion in those teratocarcinoma cell lines where HHLA1 was expressed highly [7].