When I started the process of writing a K99/R00 grant application back in July 2018, I believed that I had enough grant writing experience. I had applied for an AHA postdoctoral fellowship three different times but was not funded. I had written an R21 under the guidance of my postdoctoral PI which went through a resubmission but still missed the threshold for funding. The remnants of that R21 was eventually packaged into a more expansive R01 grant, which again went through a resubmission. But this time around it received a fundable score and my PI secured a $3 million dollar grant for the lab. All of that experience taught me three things: 1) Grant writing is a skill just like everything else. The more you work on it, the better your writing skills get. 2) Getting a grant funded is not easy. The typical batting average is .100, which means you will likely end up writing 10 grants before one is funded. 3) Getting a grant is a big deal. You should celebrate when you get one :).
The requirements for a K99/R00 are different than for an R grant, where the focus is primarily on the merits of the research plan. However, a K99/R00 grant, also called the “Kangaroo”, is a training award to help facilitate a timely transition of postdoctoral trainees to independent, tenure track faculty positions and provide NIH research support during the transition. In other words, the K99/R00 gives you 1-2 years of funding as a postdoc (K99 phase) and 3 years of funding as a PI (R00 phase), assuming you get a tenure track faculty job. In effect, you are getting $1 million dollars over 5 years to setup your own independent lab. Getting this grant is a big deal and can be a great launching point for your research career, which also makes this a highly competitive grant to get.
The eligibility criteria for a K99/R00 is a little tricky. You cannot apply, if you are more than 4 years past your terminal (Ph.D) degree. I graduated with my Ph.D. in August 2015, so my eligibility window to apply was until August 2019. The Grant application submission has only 3 cycles through the year: Feb/March; June/July, Oct/Nov. So in effect, my eligibility window would expire in June/July of 2019. The other thing to note is that they only allow for one resubmission. Moreover, if you are planning to resubmit, the 4 year window still applies. I applied in Oct 2018 but did not get funded. I submitted a revised application in July 2019, which got funded. If I had applied in Feb 2019, I would not have the opportunity to reapply until Oct 2019, but by then, my window of eligibility would have expired. Given my grant writing success rate, I knew I would have to submit a revised application and so I planned accordingly. There are obviously candidates who apply once and get funded. Alternately, there are others who are unable to get funded even after a resubmission. Even so, I would strongly recommend that you plan for a resubmission, if your timeline and eligibility window makes that option available to you.