skip orientation RF as there are not enough pairs

bwa进行外显子组比对的时候，出现以下提示

[M::mem_pestat] # candidate unique pairs for (FF, FR, RF, RR): (6, 60851, 0, 0)
[M::mem_pestat] skip orientation FF as there are not enough pairs
[M::mem_pestat] analyzing insert size distribution for orientation FR...
[M::mem_pestat] (25, 50, 75) percentile: (136, 168, 211)
[M::mem_pestat] low and high boundaries for computing mean and std.dev: (1, 361)
[M::mem_pestat] mean and std.dev: (177.36, 54.37)
[M::mem_pestat] low and high boundaries for proper pairs: (1, 436)
[M::mem_pestat] skip orientation RF as there are not enough pairs
[M::mem_pestat] skip orientation RR as there are not enough pairs
[M::mem_process_seqs] Processed 142962 reads in 35.547 CPU sec, 17.520 real sec

担心有问题，去google了下
解释如下

This is normal. The Illumina reads orientation is FR. You got majority FR and skipped other three.