We can only keep samples on hand for six months from the time of your initial order due to storage space limitations. Follow-up experiments may be performed on those samples if the request occurs within six months of sample submission. After six months, you will either need to resubmit new samples or request that your old samples be returned at the conclusion of your initial order if you think you will need them for future experiments at a later date. Shipping & handling charges apply for samples that are returned.