Excepted from the Project Website at http://www.familytreedna.com/public/R-L21/
L21 is a Single Nucleotide Polymorphism that characterizes a major R1 haplogroup. There are a number of subclades downstream of L21. An "SNP" is a Single Nucleotide Polymorphism that represents a change at a single nucleotide (A=adenine, C=cytosine, G=guanine, T=thymine) on the y chromosome. In the case of L21, that change occurred at the position known as rs11799226, where there is a change to "G" from the "C" most everyone else has.
You can trace our shared ancestry by using the YCC phylogenetic tree on at FTDNA if you are a subscriber by accessing your corresponding "myFTDNA" pages, specifically the "Haplotree" page. The ancestry of all L21+ men, in terms of SNP, runs like this:
M207+. . M173+. . M343+. . P25+. . P297+. .M269+. .L23+. .L150+. .L51+. .L11+ . .P312+ . .L21+
Anyone who has received positive results as being L21+ from Family Tree DNA may join. It is recommended that you check the bottom of your "Haplotree" page for verification. If you tested L21+ with some other company, please let me know by email so that I can advise you how the information gets included on the Family Tree DNA comparisons and reporting pages.
DO NOT JOIN THE R-L21 PLUS PROJECT UNLESS YOU HAVE AN L21+ TEST RESULT OR ARE RECRUITED FOR L21 OR DEEP CLADE TESTING OR OTHER RESEARCH PURPOSES BY AN R-L21 PLUS PROJECT ADMINISTRATOR.
BELONGING TO A HAPLOTYPE CLUSTER THAT HAS PROVEN TO BE MOSTLY L21+, OR BEING REASONABLY CLOSE TO ITS MODAL HAPLOTYPE, IS NOT ENOUGH.
There are simply too many haplotype clusters (and pseudo-clusters and dubious clusters), and too many arguments about how close to the cluster modal one needs to be to be considered part of it, for likely cluster membership to be considered a substitute for an L21+ test result.
A positive test result for an SNP (such as M222, L226, etc.) established as downstream of L21 can be inferred as the equivalent of an L21+ test result.
Although anyone with an L21+ result can join, we strongly recommend that a member have the full panoply of Y Haplogroup R SNP testing by means of Family Tree DNA's Deep Clade-R test and/or extended deep clade testing.
HOW TO JOIN: First, you MUST have an L21+ result and you must be an FTDNA customer. To join the R-L21 Plus Project, just go to your "myFTDNA" pages and click on "Join Projects" on the left side of the screen. Next, scroll down to "Y Haplogroup Projects". Click on the "R" under that heading. It has a number next to it, but that number is unimportant (it's the current number of y haplogroup R projects). On the next screen, scroll down until you find "R-L21Plus". Click on it and then on the little orange "Join" button in the lower right. That should do it.
Or click on "Join Request" at the top of the screen and follow the instructions.
You can join this project with just 12 y-dna STR markers, but 37 or, better still, 67 markers will be of more help to you and to the R-L21 Plus Project in figuring out our shared ancestry.