Hi Marc,
You should be able to copy the sequence as plain text from wherever you have it (e.g. shown in some text editor or browser window), and then paste it into the Paste area.  You would just use the normal copy/paste mechanisms of your system, e.g. command-C, command-V.  E.g. if you had a fasta file, open it in a text editor, copy the text, and then click into ChimeraX and paste it into the AlphaFold dialog Paste area.

For example, I can copy the sequence of deer LDLR from this page
<https://www.ncbi.nlm.nih.gov/protein/OWK12557.1?report=fasta>

and paste it in the AlphaFold dialog.  Clicking Fetch finds the human one in the AlphaFold database.
I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco


> On Nov 17, 2021, at 7:36 AM, Marc Pusey via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
>
> Hi -
> I may be missing something very obvious, but...  I am trying to input and use protein sequences for structure predictions using the AlphaFold feature of ChimeraX.  I've not had any problems when the sequence can be obtained from a UniProt file, but several of the proteins I'm working with do not have such a file available.  Additionally, there is going to come a time where I will want to see what the consequences are from making mutations to those sequences that are available from UniProt.  The input gives the option of either Paste or UniProt, but I've not found a way to do a simple cut and paste to put in the sequence. 
> I'm using a Mac running Mojave, version 10.14.6.
> Thanks!
> Marc Pusey