Improving PacBio RS sequence lengths. The sequence length histograms of four PacBio RS chemistries are shown using 100 bp buckets. Solid lines correspond to observed sequence lengths and dashed lines correspond to fitted log-normal distributions  with the specified mean and standard deviation. Since the initial instrument release in April 2011, the average sequence length increased over 3.5-fold through December 2012. (A) The original C1 chemistry, released in April 2011; (B) C2 chemistry, released in February, 2012; (C) XL-C2 chemistry, released in December 2012; and (D) XL-XL chemistry, released in December 2012.