After horizontal gene transfers, metabolic pathways may need further optimization: Effective use of a horizontally transferred pathway can require co-evolutionary changes in the host or pathway

Pathways acquired through horizontal gene transfer (HGT) may impose new stresses on recipient cells. HGT-related stresses can limit the utility of newly acquired genes. Effective use of horizontally transferred genes may require refining mutationsto both the recipient and the transferred genes. Synthetic biologists deliberately transfer genes between species and seek to determine how such genes can be optimized to function properly within recipient cells.