A careful Bible student writes:

In Genesis 35:23-26, the 12 sons of Jacob are listed. Dan is named there but not Manasseh. In Revelation 7:5-8 the 144,000 sealed from all the tribes of the children of Israel are named. Here Manasseh is named but not Dan. I am wondering why. Thanks for your help.

Manasseh was not a son of Jacob; instead he was "the first-born of Joseph" (Joshua 17:1), and so he was a grandson of Jacob. Some Greek copyist made a mistake by inserting the name "Manasseh" in Revelation 7:6 instead of "Dan," who was the fifth son of Jacob (Genesis 29:32-30:6; 35:22-26). Perhaps they inserted "Manasseh," because he, along with his brother Ephraim, became very prominent in Israel's history (Joshua 16:1-10; 17:1-12), more prominent than Dan. "Ephraim and Dan, being the principal promoters of idolatry, are left out in this enumeration" (Adam Clarke's comment on Revelation 7:5-8). However, a Bohairica manuscript has "Dan" in Revelation 7:6.

I noticed years ago the insertion of Manasseh (not a son of Jacob) and the omission of Dan (a son of Jacob) in Revelation 7:6, and I had written a question on the margin of my old Greek NT text in 1953: "Each tribe has a quota? Dan none?" But Dan had a quota (Joshua 19:47-48). I should have made the correction of all English translations in my translation but I overlooked it.

Jacob had tried to change the number of his sons from 12 to 14 by claiming the first two sons of Joseph, Ephraim and Manasseh (Genesis 48:1-5), as his children. However, that claim was never allowed. Long after Jacob's death only the "twelve tribes of Israel" (Luke 22:30; Acts 26:7; James 1:1) were recognized, not fourteen. We read of "the twelve patriarchs" (Acts 7:8), not of fourteen. We read of 144,000 (Revelation 7:4; 14:1), that is, 12 times 12,000, but not of 168,000, 14 times 12,000. Hence, the reference to Manasseh in nearly all of the Greek manuscripts of Revelation 7:6 has to be an error.