Psalm 145 is an acrostic of the Hebrew alphabet, except that in most of the MT manuscripts verse 13b is missing along with therefore the letter nûn. The ESV renders it like this:
Your kingdom is an everlasting kingdom, and your dominion endures throughout all generations. [The LORD is faithful in all his words and kind in all his works.] The LORD upholds all who are falling and raises up all who are bowed down.
I understand the textual evidence somewhat for 13b in the Syriac, LXX and Qumran, but I find it a bit incredulous that a scribe would accidentally make such an obvious deletion and that it would continue to be copied that way thereafter.
Is 13b (i.e. the line beginning with nûn) a deletion from the MT or an insertion in other families? And if it is an insertion, what is the significance of leaving out the one letter?