Using EXtra Long Polymerase Chain Reaction (XL PCR) conditions, targets of up to 30 kb in size have been amplified from total human genomic DNA 1). These conditions reflect our current understanding of key parameters for successful PCR amplification of long (>10 kb) targets 1–4). Specifically, the single-stranded integrity of the template DNA must be protected during sample preparation and during thermal cycling. In each cycle, template strands must be completely denatured, extension times must be sufficiently long, and nucleotide misincorporations that could cause premature termination of strand synthesis must be removed. Primer design and reaction stringency must also be optimized for successful single-copy gene amplifications from complex genomic DNA. Optimal reaction conditions may be very system dependent, particularly as target length increases. With increased understanding of the key variables and subsequent improvements in long PCR technology, increasingly longer targets will be routinely amplifiable.