ויחי יעקב בארץ מצרים שבע עשרה שנה (ויחי מז:כח)

Yaakov saw that the Aleph Beis (Hebrew letters) missing in the names of his sons are the letters “ח” and “ט,” which spell the word “חטא” – sin.  Since they do not possess any sin, they are worthy of the revelation of the end of Galus – when Moshiach will come.  However, the letters “ק” and “צ” were also missing – spelling the word “קץ” – the end – therefore, he could not reveal the end of the exile.