To further investigate the absolute configurations of C-24 and C-25 in 1-3, we attempted to crystallise 1 and 2, which was not successful, perhaps due to the flexibility of the polyene moiety. However, in polyketides, the chirality of β-hydroxy group and αsubstituent can be predicted by analysis of the conserved residues on the KR domain. We investigated the KR domain of BueA by multiple sequence analysis to identify conserved fingerprint residues (Fig. ). We observed that BueA contained the signature leucine and aspartic acid from the LDD motif of B-type β-ketoreductases, which yields an R-hydroxy group, and a glutamine characteristic of B1-type KRs, which yield an R-α-substituent configuration (Fig. ). These results suggest that 1-3 likely possess 24R,25R configurations, similar to fusarielin. However, exceptions to the rule have been observed, as in the case of the PKS Hpm8, 36 so these stereochemical assignments are only tentative at this stage.